[time-nuts] broadband MPX signal stereo

[Demian Martin]

I have tested a number of soundcards and while the EMU 2020 has issues
(serious jitter and noise from the USB interface) I can recommend the ESI
Juli@ as having flat response and good SNR up to 90 KHz. It's a PCI card, no
USB. I have measured the performance of FM MPX adapters and tested FM
Transmitter performance with one. You get 24 bit at 192 X 2 continuously as
long as you want. With some simple pre-filtering and gain you should be able
to get some 60 KHz signal at very low levels. With a deep FFT I get to -130
dBFS easily. With the demo board for the AKM AK5345a I am getting better
than -160dBFS. That has SPDIF out. Depending on the filter selection, most
chips have a flat option, there is no significant phase shift up to
frequencies close to the cutoff.

I have tried and dumped a number of other sound cards that were dead ends.
The ESI Juli@ is usually around $120. 
       Demian
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Chris Albertson writes:

Some audio interfaces have a low pass filter to cut off at about 20KHz
but many don't have the filter.     The user manual should list the
bandwidth of the interface.  Mine claims to be flat out to 40KHz.  But
it varies.

For example the EMU 2020 user manual reads
"Frequency Response (min gain, 20Hz-20kHz): +0.0/-0.07dB"
But what happens after 20KHz?  The specs don't say.   You have to test
it yourself and see.