[RTTY] Sound Cards

[Richard Ferch]

N6PE wrote:

> I just ordered a little SoftRock SDR card and have been reading about the 
> performance with onboard vs PCI 24-bit cards. The standard 16-bit 48 khz 
> motherboard sound is really a poor performer compared to a 24-bit PCI card.
> 
> I'm wondering if there is any advantage or performance improvement to be had 
> with MMTTY and other RTTY programs by using a 24-bit PCI card ? Tests done? 
> Charts etc?
>
The performance requirements for an SDR and for RTTY are totally 
different. In an SDR, the number of bits of resolution translates to 
dynamic range - the more bits, the more dynamic range. 16 bits is not 
enough for good SDR performance. RTTY, on the other hand, is supposed to 
be a constant-amplitude mode; your receiver's AGC takes care of the 
dynamic range, so even 16 bits is massive overkill for an RTTY decoder.

Another parameter that is important for an SDR is sampling frequency - 
the higher the sampling rate, the wider the bandwidth of the receiver 
(e.g. for panoramic displays or multiple signal decoders like CW 
Skimmer). RTTY's sampling rate requirements are much less onerous - 8 
kHz is more than enough.

The really important performance criterion for RTTY reception (and 
transmission) is noise, and in most situations I suspect this is 
dominated by system issues outside the sound card, such as ground loops, 
"pin 1" problems, RFI, hum pickup, poor power supply filtering, etc.

> Naturally the SDR needs stereo and the RTTY programs do not. Maybe there is a 
> program (or programmer wanting to write one) out there that uses filtered audio 
> to the R/L of a stereo card for mark/space.

It is unlikely that external filtering to separate the mark and space 
tones into two channels could do as good a job as software can already 
do with a single audio stream. However, there is a situation where 
stereo can be useful in a sound card for RTTY, and that is when you have 
two receivers. You can feed the first RX audio into the left channel and 
the second RX (subRX or second radio) audio into the right channel, set 
up two copies of MMTTY, one configured to use each channel, and decode 
signals from both receivers at once with a single sound card.

73,
Rich VE3KI