어제는 조촐히 59DX 클럽 (호출부호 DT0HH) 송년회겸 남극 세종기지 통신 엔지니어로 가시는

DS4NMJ 국장의 송별회가 있었습니다. 그래봤자. 평일이고 많은 맴버의 주서식처(?)가 서울이

아니라 3명밖에 못 모였습니다. DS4NMJ 국장이 워낙 발이 넓은지라 전국 일주를 하고 있는듯

했습니다. 남극에 가시면 온에어에 HL (한국 무선국을 이렇게 부르지요) 국이 나오면 무조건

먼저 받아주실겁니다. ^.^; 모쪼록 돌아오시는 날까지 건강하게 잘 다녀오십시요.

 서울 모처 술집, DS1NPP 와 DS4NMJ[ 초상권 문제는 슬라임에게 연락바람. -.-; ]  

저번 주말은 CQ WW Contest CW 가 있었습니다. 단파대가 전신 콩볶는 소리로 바글 바글했지요.

저는 워낙 느림보 CW’er 라 컨테스트에서는 거의 나가지 않습니다. 잘 알아듣지도 못할뿐 아니라

그렇게 빨리 치지도 못합니다. 거의 기관총 소리로 들리거든요. 드륵륵 드륵륵 특히 R 5NN …

그런데 한참을 듣고 있자니 그냥 몇번 답을 해주고 싶었습니다. 사실 교신 내용은 아주 짧기 

때문에 상대국이 짜증내지 않을 정도의 속도로 쳐주면 뭐 몇국은 할거 같더군요. 흐흐 그런데

키어가 약간 맛이 갔더군요. 그래서 집에 부품을 뒤져서 컴퓨터 키어를 만들었습니다. 뭔말이냐면

컴퓨터에서 타이핑을 해주면 ‘돈 쓰 돈 쓰’ 를 자동으로 만들어주는 거죠. 소프트웨어들은 많이

나와 있으니 그냥 컴퓨터와 무전기를 연결할 간단한 스위칭 회로만 달아주면 되죠. 한 20분정도

걸려서 – 사실 전혀 계산도 안 해본 회로라 작동할지 미지수였음 – 간단히 회로를 갖다 붙였습니다.

접지전압에 약간 문제가 있어서 이상 작동을 했지만 바로 수정해서 그럭저럭 잘 작동하더군요.

ㅋㅋㅋ 그래서 러시아 3국, 미국 2국, 일본 1국 응답해 줬습니다. 우와! 그런데 컨테스트 참여국들은

정말 빠르게 칩니다. 이제 듣기 연습을 열심히 해서 컨테스트도 자주 나가볼까 생각중입니다. ^^;

조각낸 만능기판위에 트랜지스터 C1815, 다이오드 1N4004, 저항 3.3kΩ 과
스테레오 미니플러그, 오디오선, UTP선과 9핀 D-SUB 암놈 연결하고 필름통 하우징으로 마무리…

예전에 켄우드 무전기를 사용할 때 쓰던 마이크가 있는데 이것을 현재 야에스 무전기에 사용하려니

약간의 수정 작업을 해줘야 했습니다. 그런데, 중간에 디지탈 통신용으로 사용하는 ‘리그블래스터’

라는 녀석과 같이 쓰려니 그냥 덜렁 개조만 하면 안되겠더군요. 결국 켄우드 마이크와 구형 

‘리그블래스터’ 설정은 그대로 두고, ‘리그블래스터’ 와 무전기를 연결하는 케이블만 약간 개조해

줬습니다. 마이크의 주파수 업다운 스위치는 쓰지 못하지만 스텐드 마이크를 쓴다는게 또 편하기가

이루 말할 수 없습니다.

  좌측에 켄우드 스텐드 마이크, 중간에 세미누드(?)인 녀석은 리그블래스터, 한창 작업중인 모습

얼마전 포스팅에서 말씀드렸듯이 저번 주말은 제가 속해있는 아마추어 무선사 클럽인

59DX Club 에서 대부도로 원정 운용을 다녀왔습니다. 미국 CQ Magazine 에서 주최하는

CQ Worldwide DX Contest – SSB 가 열리는 기간이라 HF 밴드 전체가 시끌시끌 했습니다.

토요일 저녁부터 월요일 오전까지 330개 정도의 무선국과 교신을 했습니다. 나라수로 보면

30개국정도 되지 않을까 싶습니다. (아직 정리를 안해서 정확하지 않습니다만)

전신으로 200여국, 음성으로 100여국을 한 거 같습니다. 안테나 (V 다이폴, 모노폴) 와

송신 전력 (100W bearfoot) 이 부실한 관계로 많은 Pileup 을 제대로 뚫지 못하고 포기한

교신도 꽤 되구요. 목표는 1000국 이었는데, 장비 환경과 태양 흑점 상태에 비하면 잘했다고

위안을 하고 있습니다. ㅠ.ㅠ  내년엔 리그와 안테나를 보강하여 좀 더 좋은 스코어를

내보고자계획하고 있습니다. 아래 사진은 익스페디션 사이트에서 찍은 사진들입니다.

같이 참여하여주신 클럽 맴버들에게 수고와 감사의 인사를 보냅니다. 더불어 내년에

남극 기지에 통신기술자로 참여하시는 DS4NMJ OM 의 무사 귀환을 기원드립니다.

(사진은 클릭하시면 좀 더 깨끗이 나옵니다.)

  야간 운용중인 DS4NMJ OM – 12월에 남극 세종 기지로 가신답니다.

  현란하고 우렁찬 교신 맨트를 날리시는 6K5TET OM

  물이 찬 서해안을 바라보고 있는 우리의 V 다이폴 안테나, 안테나가 리그보다 아래 있었다는…^^;

  DS5EVU OM, DS1NPP OM 의 합작품 무바룬 모노폴 3.5MHz, 14MHz 를 담당

  교신중인 DS5EVU OM 과 야간 운용을 위한 잠자는 DS1NPP OM

  금강산도 식후경, 식사 준비중인 슬라임 (DS1MRF)

  떠나는 날 오전 21MHz 가 열리자 밀려드는 북미국과 교신에 열중인 DS1NPP OM

  사이트에서 바라 본 물 빠진 서해안

오는 주말 (‘05.10.29~31) 제가 속해 있는 59DX Club (호출부호:DT0HH) 의 맴버들과 함께 대부도 (AS-105) 로 IOTA (Island On The Air) 원정 운용을 갑니다. IOTA 란 1985년 영국 아마추어 무선 연맹에서 시작한 프로그램으로 섬들과 교신을 많이한 무선사들에게 상장을 주는 일종의 게임입니다. 전세계적으로 IOTA 상장을 받기 위해 IOTA 운용만을 사냥(?) 하는 무선사들이 있는데요. 그런 무선사들로 부터 Pileup (어떤 무선국과 교신하기 위해 무선사들이 줄을 서있는 상황) 을 받게 되지요. 그래서 IOTA 운용의 즐거움이 있습니다. 무사 귀환을 기원해 주십시요.  

2003년 59DX 클럽 원정 운용 기념 사진중 한컷 (앞에 빨간 점퍼가 슬라임-호출부호:DS1MRF)

IOTA 로고

출처: 한국전파진흥협회 (http://www.rapa.or.kr) 정기간행물 ‘전파진흥’ 2001년 9월호

   S-UNIT DISCUSSIONS
Compiled From The Internet

ORIGINALLY POSTED ON THE FINE WA4PGM SITE

In article 
Gordon Pritchardwrote:

I’ve only heard of this 6dB/S-unit by way of *rumour*, too. I’ve followed this discussion for

a bit, butdon’t know if anyone has mentioned it already. 

The 6dB/S unit figure is covered in the 3B question pool.  It’s never been clear to me which

text isused as a refrence for any of the questions, but I presume (hope, actually) they are

taken from some

sort of reliable engineering text.  I think the pool needs a Bibliography!

Jim –KE6JPO
___________________________________________________________________________

Greetings to All that read this from VE7TMA:

The ARRL handbook refers to an old standard of  S9 calibrated at 50 microvolts RF input and

6 dB perS unit down from that reference point.

The section also clarifies this by stating in reality there is seldom two rigs that follow the

sameabsolute calibration curve considering different designs and gain variations from

band-to-band.

It is stated that the S meter is useful for relative comparisons on the same rig operating

on thesame band.

Although the accurate S meter calibration subject seems somewhat of a dead issue for the

transceivers that have been manufactured in the past it is interesting to play with the

numbersconsidering the sensitivity of a typical modern rig and the calibration reference

of S9 for a 50uVRF input.

I ran a little program on my calculator to calculate the RF input voltage levels for a  S meter

scales of 1 through 7 dB per S unit  with S9 set to 50 microVolts as a reference for each
calibrationcurve.

The following table gives the results:

            ___________/________________________
                                         dB PER S UNIT                                                                             

S mtr|    1dB       2dB      3dB      4dB      5dB       6dB      7dB
=== |    ===   ===     ===      ===     ===     ===     ====

S9   |    50.0      50.0       50.0     50.0      50.0      50.0       50.0
S8   |    44.6      39.7       35.4     31.5      28.1      25.1       22.3    
S7   |    39.7      31.5       25.1     19.9      15.8      12.6       9.98   
S6   |    35.4      25.1       17.7     12.6      8.89      6.29       4.46
S5   |    31.6      19.9       12.6     7.92      5.00      3.15       1.99
S4   |    28.1      15.8       8.89     5.00      2.81      1.58       0.89
S3   |    25.1      12.6       6.29     3.15      1.58      0.79       0.40
S2   |    22.3      9.98       4.46     1.99      0.89      0.40       0.18
S1   |    19.9      7.92       3.15     1.26      0.50      0.20       0.79
S0   |    17.7      6.29       2.23     0.80      0.28      0.099     0.035

        ____________________________________________/

                          RF  INPUT LEVEL IN MICROVOLTS RMS

It is interesting to consider the sensitivity of a typical modern HF transceiver. I checked the

sensitivity specification  for three modern SSB transceivers and found it varied from 0.20

to 0.25 uVfor a 10 dB SignalSignal + noise ratio.

If one were to consider the sensitivity specification as roughly equivalent to an S zero
thresholdthen it would appear that a calibration factor of 5 dB per S unit is the closest

round number fit tothe10 dB S/S+N sensitivity  with a calibration reference of S9=50 uV. 

The 6dB per S unit curvewouldresult in a signal to noise ratio of about 3 dB S/S+N which

most likely would be below thereceiverAGC threshold (which is typically utilized to drive

the S meter circuit) which would tendto beimpractical to implement.

A  nice quality of a 6dB/SU curve is that the RF input voltage doubles for each S unit increase

ofsignal strength (representing increase of input power of 4 times for each additional S unit)

whichwould be apractical rule-of-thumb to remember.

I included calibrations in the table down to 1 dB per S unit as the original message posted on

thissubject thread made mention of a rig with a 1 dB/SU S meter response. It is interesting

to note thatif S9 were equal to 50 uV on that scale (which it probably is not) the S/S+N ratio

of an S0 signalwould be about 48 dB which is almost equivalent to the noise performance of

atelephone circuit!

That S meter scale would be of little practical value. I have seen similar calibration

characteristicsin a multi-mode 2 meter transceiver that I own. It is reasonably well

calibrated in the SSB modebut pretty hopeless during NBFM operation.

This is a case were the S meter calibration characteristics change drastically on the same

rig whenswitched between operating modes!

I think the current technology could support an improvement in the calibration of S-meters.

For example the National SemiconductorNE604 IF amplifier ICprovides an accurate signal

strengthlogarithmic output that closely tracks the input signal level over a wide dynamic

range that couldpossibly be used for driving an S meter circuit. Considering the consistency

of surface mount andwide band technologies coupled with the ability to program multi band

calibration factors into themicroprocessor S meter firmware it should be practical for

manufactures to now provide reasonableS meter calibration accuracy.

An S meter tracking accuracy specification should be included as part of  the overall

specifications.This specification would tend to take marketing biases out of the S meter

calibration scale.

I hope the manufactures take steps to clear up this age old guess meter confusion  so that

radioamateurs have the ability to routinely measure signal strengths in a reasonably

accurate mannerin the near future.

Possibly the amateur radio community may need to lobby the manufactures to give the

issue somepriority.

73 de VE7TMA 
____________________________________________________________________________________
S-Meter microVolts vs S-Units
Based on the accepted standard of 50uV = S9

MicroVolts      S-Units
   0.2                 S1
   0.4                 S2
   0.8                 S3
   1.6                 S4
   3.2                 S5
   6.3                 S6
  12.5                S7
  25.0                S8
  50.0                S9
 158.0              S9+10dB

On The Other Hand — Here is what some others measured
Readings on my IC746 are:
S1-2 1dB
S2-3 2dB
S3-4 2dB
S4-5 3dB
S5-6 3dB
S6-7 5dB
S7-8 5dB
S8-9 5dB
s9-9+10 10dB
+10-+20 10dB
To nearest dB – checked overall by dropping from S9 to S1 – total attenuation is within 1dB of

the sum of the increments.

Pre-amp seems to have about 12dB gain – at least on 17M.

Here are measurements of my Icom 730 S meter on 40 meters with preamp out:

S1 – 2 1.4 dB
S2 – 3 1.3 dB
S3 – 4 1.6 dB
S4 – 5 2.3 dB
S5 – 6 1.8 dB
S6 – 7 3.2 dB
S7 – 8 3.1 dB
S8 – 9 4.0 dB
S9 – S9+10dB 5.6 dB
S9+10dB – S9+20dB 7.3 dB
S9+20dB – S9+30dB 6.6 dB
S9+30dB – S9+40dB 10.5 dB
S9+40dB – S9+50dB 11.3 dB
S9+50dB – S9+60dB 13.5 dB

Another reports:

There is NO standard for S units, or even for S-9. There are a bunch of suggestions, but

that’s all.The suggestion the ARRL makes varies with frequency.

Most of my receivers, when designed, tried to use about 5dB per S unit. They have various

S-9calibration points. Very few receivers attempt to use 6 dB per S unit compared to the

one’s using5dB or less.

My dozen or so receivers (Drake R4C’s, IC-751A’s, Yaesu FT1000D, Collins 75S and KWM-2,

etc)all range from about 1 or less dB per S unit at S1, to maybe 3 or 5dB per S unit near S-9. 

Maybe time forNational SemiconductorNE604 IF amplifierICProject!

출처:http://www.ac6v.com/sunit.htm

PSK31 Fundamentals


Background: The PSK31 philosophy.

PSK31 is the result of my belief that the present batch of "data" modes have left a gap in amateur radio operating, the gap that was previously filled by AMTOR or even traditional RTTY, in which two or more operators chat to each other on an open channel. Modes such as packet radio, Pactor, and others, are highly complex, are unsuited to multiway conversations, and in particular, the long block lengths introduce an unacceptable delay in the processing of text such that even normal conversation is unpleasant and quick-break question/answer sessions are impossible. The move to automated unattended message forwarding has left a gap in the person-to-person communication field, and PSK31 is an attempt to remedy this situation with a simple but efficient code structure coupled with the narrowest possible bandwidth, and with only enough error-correction to match typical typing-error rates, and with no time-consuming synchronisation, changeover, and ARQ processes.

The 31 baud BPSK modulation system used in PSK31 was introduced by SP9VRC in his SLOWBPSK program written for the EVM. Instead of the traditional frequency-shift keying, the information is transmitted by patterns of polarity-reversals (sometimes called 180-degree phase shifts). This process can be thought of as equivalent to sending information by swapping-over the two wires to the antenna, although, of course, the keying is more usually done back in the audio input into the transceiver. A well-designed PSK system will give better results than the conventional FSK systems that amateurs have been using for years, and is potentially capable of operation in much narrower bandwidths than FSK. The 31 baud data rate was chosen so that the system will just handle hand-sent typed text easily.

There is a problem with PSK keying which doesn’t show up with FSK, and that is the effect of key-clicks. We can get away with hard FSK keying at moderate baudrates without generating too much splatter, but polarity reversals are equivalent to simultaneous switching-off of one transmitter and switching-on of another one in antiphase: the result being keyclicks that are TWICE AS BAD as on-off keying, all other things being equal. So if we use computer logic to key a BPSK modulator such as an exclusive-or gate, at 31 baud, the emission would be extremely broad. In fact it would be about 3 times the baudrate wide at 10dB down, 5 times at 14dB down, 7 times at 17dB down, and so on (the squarewave Fourier series in fact)

The solution is to filter the output, or to shape the envelope amplitude of each bit which amounts to the same thing. In PSK31, a cosine shape is used. To see what this does to the waveform and the spectrum, consider transmitting a sequence of continuous polarity-reversals at 31 baud. With cosine shaping, the envelope ends up looking like full-wave rectified 31Hz AC. This not only looks like a two-tone test signal, it IS a two-tone test signal, and the spectrum consists of two pure tones at +/-15Hz from the centre, and no splatter. Like the two-tone and unlike FSK, however, if we pass this through a transmitter, we get intermodulation products if it is not linear, so we DO need to be careful not to overdrive the audio. However, even the worst linears will give third-order products of 25dB at +/-47Hz (3 times the baudrate wide) and fifth-order products of 35dB at +/-78Hz (5 times the baudrate wide), a considerable improvement over the hard-keying case. If we infinitely overdrive the linear, we are back to the same levels as the hard-keyed system.

There is a similar line of reasoning on the receive side. The equivalent to "hard-keying" on the receive side is a BPSK receiver which opens a gate at the start of a bit, collects and stores all the received signal and noise during the bit, and then "snaps" the gate shut at the end. This process gives rise to the receive-side equivalent of key-clicks, namely sidelobes on the receiver passband. So, although this "integrate-and-dump" method is 100% efficient in the task of sorting out signal from noise, it will only reject signals by 10dB at 3 times the baudrate wide and so on, the same spurious rejection figures that we got as spurious emission figures for the transmit side. The PSK31 receiver overcomes this by filtering the receive signal, or by what amounts to the same thing, shaping the envelope of the received bit. The shape is more complex than the cosine shape used in the transmitter: if we used a cosine in the receiver we end up with some signal from one received bit "spreading" into the next bit, an inevitable result of cascading two filters which are each already "spread" by one bit. The more complex shape in the receiver overcomes this by shaping 4 bits at a time and compensating for this intersymbol interference, but the end result is a passband that is at least 64dB down at +/-31Hz and beyond, and doesn’t introduce any inter-symbol-interference when receiving a cosine-shaped transmission.

Note that the transmitter and receiver filters have to be "matched" to each other for the ISI performance to be right. Some systems like this use a pair of identical receive and transmit filters which are matched. If I did this and someone else came along wanting to improve the performance, they would have to get everyone else to change their transmit filters. I have therefore chosen to use the simple cosine shape for the transmitter and match that in the receiver. This leaves the way open for others to develope better receivers without new transmitters being incompatible with old. This is slightly different from the SP9VRC approach.

To summarise: PSK31 has been designed not only to give all the weak-signal-in-white-noise advantages that PSK has to offer, but to go further and optimise the performance in the presence of other signals, to reject them on receive and not to interfere with them on transmit. PSK31 is therefore ideally suited to HF use, and would not be expected to show any advantage over the hard-keyed integrate-and-dump method in areas where the only thing we are fighting is white noise and we don’t need to worry about interference.

The QPSK mode

In December 1997, PSK31 introduced the QPSK mode. In this mode, instead of just keying by phase reversals, that is, 180-degree phase-shift, an additional pair of 90 and 270 degree phase-shifts are possible. If you thought of BPSK as reversing the polarity of the signal, then QPSK can be thought of as two BPSK transmitters on the same frequency but 90 degrees out of phase with each other. By thinking of the receiver as being two BPSK demodulators at 90 degrees, we have two channels sharing the same frequency, but of course, with only half the transmitter power in each. We therefore have twice the bit-rate but at 3dB less signal-to-noise ratio. We could use this feature to transmit data at twice the speed with 3dB less noise margin.

The PSK31 philosophy is to stay at the speed needed to handle hand-keyed text, so why do we condider QPSK at all? The answer is that we can use the extra capacity to reduce the error-rate while keeping the bandwidth and the traffic speed the same. Note that because we have a 3dB SNR penalty with QPSK, any error-correction scheme we introduce has to be at least good enough to correct the extra errors which result from the 3dB SNR penalty, and preferably a lot more, or it will not be worth doing. By doing simulations in a computer, and tests on the bench with a noise generator, it has been found that when the bit error-rate is less than 1% with BPSK, it is much better than 1% with QPSK and error-reduction, but when the BER is worse than 1% on BPSK, the QPSK mode is actually worse than BPSK. Therefore, if we are dealing with radio paths where the signal is just simply very noisy, there is actually no advantage to QPSK at all!

However, all the tests we have done on the air show that QPSK with the chosen error-reduction scheme is better than BPSK, except where we have deliberately attenuated the signal to make it artificially weak. Typical radio circuits are far from being non-fading with white noise. Typical radio paths have errors in bursts rather than randomly spread, and error-reduction schemes can give useful benefits in this situation in a way that cannot be achieved by anything we can do in the linear part of the signal path. With the code used in PSK31, a 5:1 improvement is typical, but it does depend on the kind of path being used. For this reason it is worth keeping both modes available and remembering that there may be times when one mode works betterthan the other and others when the reverse will be the case. When comparing PSK31 with other modes, remember that the switch between "straight" and "error-corrected" modes in PSK31 is done with both the bandwidth and the data-rate remaining the same. In most other systems that can switch, either the bandwidth or the data rate changes when the system switches, and the figures for error-rate improvement can be misleading unless they are carefully compared.

The error-reduction code chosen is one of a type known as convolutional codes. The code systems used in the past have been block codes, where each character is a fixed-length code, and a fixed number of extra bits are added to make a longer block, and this longer block is capable of correcting errors within itself. These extended blocks are then transmitted as a serial bitstream. In a convolutional code, the characters are converted to a bitstream and then this bitstream is itself processed to add the error-reduction qualities. There is no relationship between the boundaries between characters and the error-reduction process. Since the channel errors are also not related in any way to the character boundaries, convolutional codes are better suited to serial links than block codes, which were originally designed for protecting errors in memory banks and similar structures.

It is not quite correct to refer to the convolutional code system as "error-correcting", since the raw data is not actually transmitted in it’s original form and therefore it makes no sense to talk about it being corrupted by the link and corrected in the decoder. In PSK31, the raw data is transformed from binary (1 of 2) to quaternary (1 of 4) in such a way that there is a precisely known pattern in the sequence of quaternary symbols. In the code used in PSK31, the pattern of quaternary symbols is derived from a run of 5 consecutive data bits. For example, if we label the four phase-shifts as A, B, C, and D, and suppose that the transmitter sends continuous A’s when the raw datastream is sending continuous 0’s. Because the convolutional encoder works on a run of five bits, when the datastream sends ..000010000…, the transmitter actually sends ..AAAADCCBDAAAA…,  that is, each binary bit to be transmitted results in a unique 5-symbol sequence, overlapping with the sequences from adjacent bits, in a predictable way which the receiver can use to estimate the correct sequence even in the presence of corruptions in parts of the sequence.

The decoder, known as a Viterbi decoder after the man who thought of it, is not really a decoder at all, but a whole bank of parallel encoders, each fed with one possible "guess" at the transmitted data sequence. The outputs of these parallel encoders are all compared with the received symbol-stream. Each time a new symbol is received, the encoders need to add an extra bit to their sequence guesses and consider that the new bit might be a 0 or a 1. This doubles the number of sequence guesses, but a clever technique allows half of all the guessed sequences to be discarded as being less likely than the other half, and this means that the number of guesses being tracked stays constant. After a large number of symbols have been received, the chances of a wrong guess at the first symbol tends to zero, so the decoder can be pretty sure that the first bit was right and it can be fed to the output. In practice this means that the decoder always outputs decoded data bits some time after they have been received. This delay in PSK31 is 20 bits (640mS) which is long enough to make sure that the decoder has done a good job, but not so long that it introduces an unacceptable delay in displaying the received text.
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Information Coding: Varicode

This is a description of the variable-length coding used in the 31.25 baud BPSK system.

The normal asynchronous ASCII coding used on the original version of this system by SP9VRC, and indeed the asynchronous system used for transmission of RTTY for the last 50 years, uses one start-bit, a fixed number of data-bits, and one or more stop-bits. The start-bit is always the opposite polarity to that of the stop-bit. When no traffic is being sent the signal sits in stop polarity. This enables the receiver to start decoding as it receives the edge between the stop-signal and the start-bit.

One disadvantage of this process is that if, during a long run of traffic, an error occurs in either a stop-bit or a start-bit, the receiver will lose synchronisation, and may take some time to get back into sync, depending on the pattern of following characters: in some situations of repeated characters the receiver can even stay in a false sync. for as long as the repeated pattern persists.

Another disadvantage of this system arises when, as will be the case for normal amateur radio contacts, the traffic being sent consists of plain language. In all languages there are some characters which occur more often that others and there are some which may hardly ever be used. In morse code this is used to advantage by using short codes for the common letters and longer codes for less-common ones. In the asynchronous start-stop system all characters are neccesarily the same length, and so the overall speed of transmission of plain-language is not as fast as a variable-length code would be.

The variable-length code used in the BPSK system overcomes both these disadvantages, and works in the following way.

1. All characters are separated from each other by two consecutive 0 bits.
2. No character contains more than one consecutive 0 bit.

It follows from this that all characters must begin and end with a 1.

With such a code, the receiver detects the end of one code and the beginning of the next by detecting the occurence of a 00 pattern, and since this pattern never occurs inside a character, the "loss of sync" problem that occurs with asynchronous systems can never occur. The 00 gap between characters is equivalent to the gap between letters in morse code in this respect, and in a similar way allows the possibility of a variable-length code system.

The variable-length coding used in the BPSK system was chosen by collecting a large volume of English language ASCII text files and analysing them to establish the occurrence-frequency of each of the 128 ASCII characters. Next a list was made of all the binary patterns that meet the above rules, namely that each pattern must start and end with a 1, and must not contain more than 1 zero in a row. This list was generated by computer, starting at the shortest. The list was stopped when 128 patterns had been found. Next the list of ASCII codes, in occurence-frequency order was matched to the list of binary patterns, in length order, so that the most frequently-occuring ASCII codes were matched to the shortest patterns, and that completed the variable-code alphabet. To finish the job, a simple calculation was made to predict the average number of bits in typical plain language text transmitted by this code, taking into account the 00 gap between characters. The result was between 6 and 7 bits per character. This compares very favourably with 9 bits per character for the asynchronous system.

The actual alphabet is shown below, shown in ASCII order starting with NUL and ending with DEL.

NUL 1010101011
SOH 1011011011
STX 1011101101
ETX 1101110111
EOT 1011101011
ENQ 1101011111
ACK 1011101111
BEL 1011111101
BS  1011111111
HT  11101111
LF  11101
VT  1101101111
FF  1011011101
CR  11111
SO  1101110101
SI  1110101011
DLE 1011110111
DC1 1011110101
DC2 1110101101
DC3 1110101111
DC4 1101011011
NAK 1101101011
SYN 1101101101
ETB 1101010111
CAN 1101111011
EM  1101111101
SUB 1110110111
ESC 1101010101
FS  1101011101
GS  1110111011
RS  1011111011
US  1101111111
SP  1
!   111111111
"   101011111
#   111110101
$   111011011
%   1011010101
&   1010111011
‘   101111111
(   11111011
)   11110111
*   101101111
+   111011111
,   1110101
–   110101
.   1010111
/   110101111
0   10110111
1   10111101
2   11101101
3   11111111
4   101110111
5   101011011
6   101101011
7   110101101
8   110101011
9   110110111
:   11110101
;   110111101
<   111101101
=   1010101
>   111010111
?   1010101111
@   1010111101
A   1111101
B   11101011
C   10101101
D   10110101
E   1110111
F   11011011
G   11111101
H   101010101
I   1111111
J   111111101
K   101111101
L   11010111
M   10111011
N   11011101
O   10101011
P   11010101
Q   111011101
R   10101111
S   1101111
T   1101101
U   101010111
V   110110101
X   101011101
Y   101110101
Z   101111011
[   1010101101
   111110111
]   111101111
^   111111011
_   1010111111
.   101101101
/   1011011111
a   1011
b   1011111
c   101111
d   101101
e   11
f   111101
g   1011011
h   101011
i   1101
j   111101011
k   10111111
l   11011
m   111011
n   1111
o   111
p   111111
q   110111111
r   10101
s   10111
t   101
u   110111
v   1111011
w   1101011
x   11011111
y   1011101
z   111010101
{   1010110111
|   110111011
}   1010110101
~   1011010111
DEL 1110110101
 

Contact Information

The source code which you may have with this distribution is freeware, provided it is used only for amateur purposes. If you have suggestions for improvements, or you find bugs, please report them back to me and do not broadcast your own modifications or bug-fixes

Peter Martinez G3PLX
High Blakebank Farm
Underbarrow
Kendal
Cumbria LA8 8BN
United Kingdom

peter.martinez@btinternet.com 
 
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