From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jonathan Naylor Subject: Re: JT44 on Linux Date: Wed, 14 Aug 2002 15:05:32 +0200 Sender: linux-hams-owner@vger.kernel.org Message-ID: <02081415053208.00798@g4klx> References: Reply-To: g4klx@qsl.net Mime-Version: 1.0 Content-Transfer-Encoding: 7BIT Return-path: In-Reply-To: List-Id: Content-Type: text/plain; charset="us-ascii" To: Tomi Manninen OH2BNS Cc: linux-hams@vger.kernel.org Hi Tomi > FWIW you could take a look at my gMFSK. While I don't claim it's > necessarily very well coded or anything, it could serve as some sort > of basis for a Linux JT44 implementation. At the least there might be > some code that can be reused. > > Do you have a pointer to the mode specification. I would like to > take a look. I've downloaded the user-mode soundmodem code which includes yours, and will use it heavily as a basis. The AFSK modem is great as a basis for the TX side, in fact JT44 is simpler, no scrambling, no bit-stuffing, etc. From the soundmodem code I can also get the soundmodem interface code, the PTT interface code and probably other things also. I will cut & paste a document into this e-mail which was the original JT44 specification document. A more modern description is available as a PDF from Joe's web pages at http://pulsar.princeton.edu/~joe/K1JT/WSJT222.PDF In some ways the older description says more about the internals of the WSJT implementation, although the newer document has more details and you really need both for an implemenation. I've already started on the TX sidem with a simple command line based program to test some of the concepts. I have a friend who has JT44 receive capability who is over a variable 250kms 23cms path which would be an interesting test. I can give you the URL of a web page with .wav files of JT44 signals at various strengths if you are intersted. Jonathan HB9DRD/G4KLX Development Notes on JT44 ------------------------- Joe Taylor, K1JT March 28, 2002 I have not yet committed a technical description of the JT44 protocol to paper. It will be posted here as soon as it becomes available. In the meantime, for the curious, here are some notes on the essentials. Some familiarity with the WSJT program will be necessary for full understanding of what follows. 1. Transmit and receive periods are nominally 30 sec each, starting on UTC half-minutes. JT44 is a time-synchronized communication mode, and in WSJT the only way to transmit or receive it is to set the program to "Auto Mode ON". 2. Transmit audio starts 1 second into the TX interval and lasts for 135 * 2048 samples at the 11025 Hz soundcard sampling rate, or about 25.08 seconds. 3. The last 3.9 seconds (minus necessary relay switching time, etc.) of the transmit period will probably be used for a fast CW ID. (This function is not yet implemented in WSJT v1.9.4). The idle time also serves to accommodate EME propagation delays. 4. The message format involves 135 intervals of data transmission, each 2048 samples long. Of these, 69 intervals carry a synchronizing tone at frequency 118*11025/1024=1270.5 Hz (approximately). 5. The remaining 66 intervals carry tones at frequencies (120+N)*11025/1024, with 1 <= N <= 43. The value of N conveys the character code. Permissible characters include the digits 0-9, letters A-Z, and special characters .,/#?$ and . The 66 character intervals carry a 22-character message, repeated three times. 6. The 69 sync-tone intervals and 66 character-tone intervals are interleaved according to a pseudo-random pattern having the desirable property that its auto-correlation function has a single spike at lag zero and falls to low values everywhere else. Detecting and aligning with this sync-tone pattern is one of the the main "secrets" of JT44, allowing the software to accommodate large frequency and clock errors. 7. At present the program synchronizes reliably with frequency errors in the range +/- 600 Hz and clock offsets from -2 to +4 seconds. The time range was made asymmetrical so as to accommodate EME delays. 8. The cost of using about half of the transmission time for the sync tone is approximately 1.5 dB. This seems to be a very good compromise in practice. It means that transmissions will "sync up" reliably at the receive end even when the S/N is -25 dB relative to the system noise in a 2500 Hz bandwidth. Note that by comparison, the minimum CW signal strength that can be copied is about -11 dB relative to same noise level. JT44 can get through with solid copy whan you cannot even hear the other station's signals. 9. Single letters in the 22-character message will have worse signal-to-noise ratios than that of the sync tone by a factor equal to the square root of 69/3, or 6.8 dB. However, that loss can be made up by averaging the received character-tone spectra over many 30-second reception periods. For such incoherent averaging, each doubling of the number of periods buys you 1.5 dB in S/N. Four periods gets you 3 dB improvement, 16 periods gets 6 dB, and so on. If the signal strength remains fairly steady, these numbers mean that good copy of any reliably synchronizable message can be achieved in about 15-20 minutes. 10. See also the accompanying file EXAMPLE.TXT, which describes an example of JT44 usage.