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#12
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Ok, I have one more additional question. :-)
For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the incoming bits? After, say, the 30th bit value, how does the receiver know that it *IS* the 30th bit value? Especially with three 1's or three 0's consecutively and no frequency changes...? Is the receiver just very accurately timed? When it occurs, do the transitions from 0's to 1's (and vice versa) serve to resynchronize the receiver with the transmitter? Sorry for the storm of questions, but thanks in advance! Dave |
#13
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Ok, I have one more additional question. :-)
For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the incoming bits? After, say, the 30th bit value, how does the receiver know that it *IS* the 30th bit value? Especially with three 1's or three 0's consecutively and no frequency changes...? Is the receiver just very accurately timed? When it occurs, do the transitions from 0's to 1's (and vice versa) serve to resynchronize the receiver with the transmitter? Sorry for the storm of questions, but thanks in advance! Dave |
#14
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----- Original Message -----
From: "David Harper" Subject: FSK technical question For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the OK, remember the whole start and stop bit thing? The line sits at mark when idle. When a character comes, the line drops to space for one bit time. This is the start bit. Then the 5 or 8 bits are transmitted, then one, 1.5 or two stop bits, which are really nothing more than the minimum time between characters. So the receiver is guaranteed *at least* one bit time of mark followed by exactly one bit time of space between characters. The receiving side does need to be reasonably accurate, but only accurate enough to not garble a character. It never has to keep in sync for more than 10 bits worth (8 data bits plus a start and stop bit). If the protocol specifies more than one stop bit, from the receiver's perspective that is nothing more than additional time the transmitter has allotted to do end of character processing. On your earlier question about receiving FSK, the various posters answered what you would do if you wanted to use an SSB or AM rig, or an audio FM rig to receive FSK. However, a purpose-built FSK receiver would probably use an FM discriminator, and simply recover data, rather than audio, from the discriminator. Remember that an FM discriminator has an output that is related to the frequency. If you fed the discriminator two frequencies, the output would be two voltages. ... |
#15
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----- Original Message -----
From: "David Harper" Subject: FSK technical question For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the OK, remember the whole start and stop bit thing? The line sits at mark when idle. When a character comes, the line drops to space for one bit time. This is the start bit. Then the 5 or 8 bits are transmitted, then one, 1.5 or two stop bits, which are really nothing more than the minimum time between characters. So the receiver is guaranteed *at least* one bit time of mark followed by exactly one bit time of space between characters. The receiving side does need to be reasonably accurate, but only accurate enough to not garble a character. It never has to keep in sync for more than 10 bits worth (8 data bits plus a start and stop bit). If the protocol specifies more than one stop bit, from the receiver's perspective that is nothing more than additional time the transmitter has allotted to do end of character processing. On your earlier question about receiving FSK, the various posters answered what you would do if you wanted to use an SSB or AM rig, or an audio FM rig to receive FSK. However, a purpose-built FSK receiver would probably use an FM discriminator, and simply recover data, rather than audio, from the discriminator. Remember that an FM discriminator has an output that is related to the frequency. If you fed the discriminator two frequencies, the output would be two voltages. ... |
#16
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"David Harper" wrote in message
m... Ok, I have one more additional question. :-) Sorry, I skipped something on the previous response. I answered for ASYNCHRONOUS serial such as RTTY or async ASCII. Some protocols, such as packet, use SYNCHRONOUS serial. Synchronous serial is a lot harder to receive. There are no start and stop bits, so the protocol doesn't involve that part of the overhead that async uses. There are several synchronous protocols, but they mostly involve two characteristics.... first, there is some mechanism for the receiver to recover the clock. Frequently, the clock is embedded in the data, although is could be sent over another channel. This allows the receiver to know the bit boundaries. Every so often (typically every data packet) a special pattern is sent that allows the receiver to identify the character boundaries. In the common protocols, such as X.25 (or AX.25), there is also a prohibition against sending too many of the same bit in a row. Special procedures are invoked if this happens in the data. ... |
#17
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"David Harper" wrote in message
m... Ok, I have one more additional question. :-) Sorry, I skipped something on the previous response. I answered for ASYNCHRONOUS serial such as RTTY or async ASCII. Some protocols, such as packet, use SYNCHRONOUS serial. Synchronous serial is a lot harder to receive. There are no start and stop bits, so the protocol doesn't involve that part of the overhead that async uses. There are several synchronous protocols, but they mostly involve two characteristics.... first, there is some mechanism for the receiver to recover the clock. Frequently, the clock is embedded in the data, although is could be sent over another channel. This allows the receiver to know the bit boundaries. Every so often (typically every data packet) a special pattern is sent that allows the receiver to identify the character boundaries. In the common protocols, such as X.25 (or AX.25), there is also a prohibition against sending too many of the same bit in a row. Special procedures are invoked if this happens in the data. ... |
#18
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In (rec.radio.amateur.digital.misc), David Harper wrote:
Ok, I have one more additional question. :-) For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the incoming bits? After, say, the 30th bit value, how does the receiver know that it *IS* the 30th bit value? Especially with three 1's or three 0's consecutively and no frequency changes...? Is the receiver just very accurately timed? When it occurs, do the transitions from 0's to 1's (and vice versa) serve to resynchronize the receiver with the transmitter? I don't view it as a storm of questions; I'd be surprised if anyone did, considering the floods asked by folks in some other newsgroups. Synchronization can be A Right Bitch. Good, Cheap Timing is part of the answer, and I think that the receivers also do some timebase adjustments as needed to keep their bit-rate clocks in sync with the transmitters'. When you add start and/or stop bit, things get a lot easier, and that is the case with most serial communications: you can reset the character and bit-time clocks per-character. When no sync bits are present, you have to derive the bit timing and character timing from the data-bit transitions in the data stream, and things can get a bit iffy. Telco circuits have hardware that requires K transitions per N bit times, and will stuff "1" or "0" bits into the stream on one end, and delete them on the other, before they get to the customer gear, so that the stream appears to be synchronous, even though it isn't really synchronous inside the telco circuit. But TY gear is _asynchronous_: it has bits to signal the start and end of each character. The general structure of a TTY character is Start_Bit, Data_Bits, Stop_Bit. The Start_Bit tells the machinery that there's a character coming down the pipe, and that it should get ready to move. When I was doing military communications, Way Back When, the start bit was a 1.5 bit time MARK, since there really were parts that had to get ready to move, clutches to engage, and so on, and the extra time ensured that things were ready when the first data bit came in. The stop bit was a 1.0 bit SPACE, IIRC, so that there was always a polarity change to signal a new character. But that's memories almost 40 years old, and I Could Be Wrong. Try this for more info: http://www.repairfaq.org/filipg/LINK/PORTS/F_The_Serial_Port1.html#THESERIALPORT_008 -- Mike Andrews Tired old sysadmin |
#19
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In (rec.radio.amateur.digital.misc), David Harper wrote:
Ok, I have one more additional question. :-) For a communications protocol such as RTTY, I know the mark and space frequencies indicate 0 and 1 values of a (usually) 5-bit character. But how does the receiving side synchronize with the transmitting side? How does the receiver continue to properly allocate the incoming bits? After, say, the 30th bit value, how does the receiver know that it *IS* the 30th bit value? Especially with three 1's or three 0's consecutively and no frequency changes...? Is the receiver just very accurately timed? When it occurs, do the transitions from 0's to 1's (and vice versa) serve to resynchronize the receiver with the transmitter? I don't view it as a storm of questions; I'd be surprised if anyone did, considering the floods asked by folks in some other newsgroups. Synchronization can be A Right Bitch. Good, Cheap Timing is part of the answer, and I think that the receivers also do some timebase adjustments as needed to keep their bit-rate clocks in sync with the transmitters'. When you add start and/or stop bit, things get a lot easier, and that is the case with most serial communications: you can reset the character and bit-time clocks per-character. When no sync bits are present, you have to derive the bit timing and character timing from the data-bit transitions in the data stream, and things can get a bit iffy. Telco circuits have hardware that requires K transitions per N bit times, and will stuff "1" or "0" bits into the stream on one end, and delete them on the other, before they get to the customer gear, so that the stream appears to be synchronous, even though it isn't really synchronous inside the telco circuit. But TY gear is _asynchronous_: it has bits to signal the start and end of each character. The general structure of a TTY character is Start_Bit, Data_Bits, Stop_Bit. The Start_Bit tells the machinery that there's a character coming down the pipe, and that it should get ready to move. When I was doing military communications, Way Back When, the start bit was a 1.5 bit time MARK, since there really were parts that had to get ready to move, clutches to engage, and so on, and the extra time ensured that things were ready when the first data bit came in. The stop bit was a 1.0 bit SPACE, IIRC, so that there was always a polarity change to signal a new character. But that's memories almost 40 years old, and I Could Be Wrong. Try this for more info: http://www.repairfaq.org/filipg/LINK/PORTS/F_The_Serial_Port1.html#THESERIALPORT_008 -- Mike Andrews Tired old sysadmin |
#20
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Another little piece of the story is that in start-stop operation the
receiver samples the incoming signal at the place where it expects the center of the bit to be. Thus it is tolerant of signals that are too fast or too slow. Of course today it is easy to get the speed very precise; but in the early days it was a matter of motors with centrifugal speed governors. With synchronous transmission the receiver knows where the bit boundaries are going to be, so it is possible to sample near the end of each bit time when all of the energy in the received signal has come in. Start-stop has to throw away roughly half of the energy in each bit because of the center sampling. Hence synchronous transmission has an advantage in signal-to-noise ratio. In the days of mechanical teleprinters, synchronous operation had a much greater advantage. A mutilated STOP pulse would allow the receiving shaft to continue rotating, and then several characters would be received in error as a result of that single bit error. With electronic reception there is no rotating shaft, so it is possible to reset the receiver to the starting position instantly. It is also possible with electronics to achieve a quasi-synchronous operation with start-stop signals. The idea is that instead of having the STOP pulse be arbitrarily long, it is of fixed length and an idle character is sent if there is nothing to send from the keyboard. This is usually called "diddle". With the incoming data stream being a steady stream of printable characters and nonprinting idle characters it is synchronous for the duration of the transmission. The detector can synchronize to this signal and take advantage of all of the energy in each signal pulse. The K6STI RITTY software (no longer marketed) operates on this principle. -- jhhaynes at earthlink dot net |
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