[RTTY] RTTY Radio IC7600 vs IC700 vs FT5000

[Kok Chen]

On Apr 8, 2011, at 9:24 AM, Stein Roar Brobakken wrote:
> Is there anyone out there which have compared or tested a FT5000 on RTTY ?

Do you have a tough RF environment?  (for example, another contester within 1 km of your station?)  If so, I would suggest looking at the FT-dx5000.  Under such a situation, receive IMD can be a problem, and Rob NC0B has a nice table showing how each rig performs (his table is listed in order of IMD dynamic range):

http://www.sherweng.com/table.html

The 5000 has two independent receivers, but you cannot quite do SO2R (in spite of what the QST review claims) with just the rig by itself.  However, the second receiver allows split DXing to be  performed on software that implements two receivers, just like on the FT-1000MP. 

(However, if you use QSK CW through a paddle, you might want to hold off buying one until Yaesu has fixed the "short first dit" problem for sure -- many people still have to ship their FT-5000 back to get that problem fixed, and for a rig that weighs 20 kg, it is not something you really want to do.)

> I know ICOM IC7600 have DUAL peak filter for RTTY, but people complained over too many buttons to touch compared to the PRO III.

"Dual peak" filters are the obvious case where you should be viewing the rig's I.F. filters as roofing filters for the sound card (even if they are not "roofing" filters for the I.F. stages).  There is relatively low energy between the two tones, so any attenuation there that is provided by a "dual peak" filter is negligible in terms of saturating the sound card.

As such, optimal filtering for digital modes is better achieved in the software modem than by using approximations in the DSP stages of the rig.  And software UI can be simplified to anything that you wish.

With the processors in today's computers, there are not that may things DSP processors can do in real time that the computers cannot also in real time, where latency is not a problem (the exception is the decimation process of the direct SDR that starts at 70 MHz).  

A dual peak filter (usually done in the DSP stages of a rig) is just an approximation of two narrow bandpass filters around each of the two RTTY tones.  This is what produces the "depression" between the two tones.  It approximates what a pair of matched filters does in software that supports matched filtering.  In fact, one thing to note is that if the software already implements matched filtering, engaging a dual peak filter will make copy worse, not better.  If copy gets better, it means that the software filter is not optimal.  

A simple dual peak filter cannot track the baud rate of the RTTY signal (at least I have not seen one that does).  I think RITTY's matched filter is also fixed at 45.45 baud, but there are other software that implements matched filtering which tracks the baud rate when you change it.  In the case of RTTY, a matched filter has an impulse response that corresponds to the rectangular pulse of the demodulated RTTY signal, i.e., a pair of sin(x)/x shapes in the frequency domain.

73
Chen, W7AY