--- In digitalradio@yahoogroups.com, "Peter G. Viscarola" < > On the topic of sound cards for digital modes in general: > > While using a sound card with a wide frequency response, > and using high sampling rates, might intuitively appear to be > a good idea, I'd suggest this is not likely to be the case. > > Given that we tend to use the sound card to monitor a radio passband > that's not (likely to be) more than 5KHz wide, sampling much > above the Nyquist frequency for 5K (10K samples/second) seems > counter-productive.
The Nyquist (and Shannon!) theorem is often mistunderstood in a manner as demondtrated above. The theorem establishes a *minimum* sampling rate to reliably reconstruct a baseband signal (and not a maximum!). For more on this, see: http://en.wikipedia.org/wiki/Nyquist-Shannon_sampling_theorem A 5khz sinewave sampled at 10 kHz, as mentioned in the example above, will be reconstructed (either in memory to feed applications or to, say, a DAC) as a square wave. At 48 or 96 kHz sampling rate, the resulting representation of the sinewave will look a whole lot more like the original sine wave than the one sampled at 10 kHz. > It seems to me that the key criteria for choosing a sound card for > digital use would be: > > - Flat frequency response from (some low frequency such as) 100HZ to > 5KHZ Every card has this property. > - Dynamic range in the area of 100db (and hence, a very low noise > floor) The noise floor of what? Dynamic range is defined by the sample size, the number of bits per sample. For digitized sound, the dynamic range is 6dB per bit. In order to achieve 100dB dynamic range, you need at least 16 bit samples. > Additionally, it seems like it would probably be a good idea if the > sound card had filtering to remove frequencies below 100Hz and above > 5KHz. There's no need for the sound card to do so. This is already done in the radio. A flat response is not as necessary for a narrow bandwidth mode like PSK31 or CW, but it is beneficial for wide bandwidth modes. People should be much more worried about a flat passband curve of the IF filters than the frequency response of the sound card. The latter is usually extremely flat, while the former is usually not. To see some passband curves you can check out a web page of mine that shows some of these: http://www.kr1st.com/FT897filter.htm If you look at the IC-718 curve you can see that there is easily a 3dB difference between the 1000Hz and 1500Hz. Even though that may not seem much, it is a power difference of 2! If you'd transmit a two tone digital mode at those frequencies then your tones would be seriously out of balance, resulting in perhaps a 15 Watt signal at 1000Hz and 30 Watts at 1500Hz, if no ALC would be present. Since there is an ALC in almost all radios, you actually have to take the response curve of the ALC into account, too. And then we haven't even touched the linearity of the finals.... So the moral of the story is to worry more about your rig than your soundcard. :-) Whatever you have will probably be fine. 73, --Alex KR1ST http://www.kr1st.com.
