[RTTY] FLDigi vs MMTTY?

Kok Chen chen at mac.com
Fri Jun 18 15:31:28 PDT 2010


On Jun 18, 2010, at 2:56 PM, Kok Chen wrote:

> The Windows multimedia timer is specified with a granularity of 1 ms (http://msdn.microsoft.com/en-us/library/dd757633(v=VS.85).aspx).

By the way, there is a potential of jitter even when you use on-off keyed tones, since the rising edge and trailing edge of a tone burst might not be aligned to the actual position of a Baudot bit.  And the tone decoder design itself could add jitter. 

I believe fldigi uses a tone at 1 kHz, but I don't know if it always start a keyed tone with a zero phase angle.

cocoaModem uses a 2 kHz tone, and the phase of a tone burst does get reset before it starts, so the leading edge lag is a constant.

Notice that jitter is a bigger problem at 75 baud (13.33 ms bit period) than at 45.45 baud (22 ms bit period).

So, how much does 1 ms matter with a 13.33 ms bit period?  About a third of a dB if the receiver is using a matched filter.  

Again, I refer to Alex VE3NEA's plots at http://www.dxatlas.com/RttyCompare/ ... right at the threshold of decoding in an AWGN channel, if represents a difference between for example, getting a 7% error rate versus getting a 10% error rate.  

If you already have clean copy, it won't matter at all.  A third of a dB also doesn't matter when there is selective fading or flutter since the bit error curve flattens out.  You can also recover 1/3 dB by raising RF output power by about 8%.

In short, a 1 ms error is no big deal for us slow pokes.  But if the error gets to 5 ms, it would become significant.

It is also not a problem for things like MFSK16 and DominoEX since the baud rate (where timing comes into play) is only about 1/4 of the bit rate.  But then, 100% of those guys uses "AFSK," anyway.

73
Chen, W7AY



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