> I'm interested in an evaluation of the sound processor in the various
> interfaces. Those can be evaluated without regard to people's tastes.
The difference isn't in the sound chip in the various interfaces.
Except for microHAM microKEYER II and Digikeyer II, *all* of the
amateur products use chips from the TI PCM290xx family. The PCM29xx
are all single input (either Line or microphone) with 16 bit analog
to digital converters.
microHAM microKEYER II and Digikeyer II use chips from Micronas with
separate mono mic and stereo line inputs. However, again the Micronas
chips are 16 bit analog to digital converters.
The real difference among the sound card interfaces is the care with
which the designer handles issues like filtering the USB power (all
of the sound cards are USB powered), bypassing noise generated by the
digital chips, *regulating* the reference voltage to the ADC, and
providing isolation between the analog (USB) power return and digital
(control) power return.
A 16 bit ADC can provide about 96 dB of theoretical dynamic range. The
best of the interfaces will achieve in the high 80 dB range. The worst
(the least expensive interfaces) that have poor filtering of USB power
and do not regulate the ADC reference will display dynamic ranges in
the low to mid 70 dB range.
The second key is whether the audio input and output are "passive"
(passive circuits between the rig and ADC/DAC) or "active" (amplified).
The "low quality" interfaces will have either passive circuits or
the amplifiers will not be optimized for low noise and distortion.
The better interfaces will use low noise, low distortion op amps on
the input (the "Line Out" of most rigs is in the 50 to 100 mV range
where the ADC expects to see ~ 4V P-P!) and low distortion op amps
on the output capable of providing 2 to 3V P-P at 50 to 75% "digital
gain" to insure clean drive to the "Packet", "Accessory" or "Data"
input of most rigs.
The other key is the choice of isolation transformers (although there
is an argument whether then are really necessary if the audio returns
are handled properly). The best interfaces use quality transformers
like the Bourns LM-NP-1001-B1 which are quite resistant to overload
(clipping) and have a wide bandwidth (to 100 KHz in a good design). The
"low price/low quality" designs often use inexpensive transformers
designed for narrow band modem use that suffer from saturation on
strong signals and significant low frequency roll off.
Bottom line, the best "evaluation" of sound card devices is to inspect
the circuit board. Look for liberal use of bypass capacitors on all
digital and analog power supply lines, make sure the ADC reference is
regulated (as recommended by the manufacturer's data sheet) and look
at the quality of the transformers.
microHAM's interfaces use "dual" power - USB power for the "sound card"
(and USB) interface with rig or external +12V for the microprocessor
(for those interfaces that use a coprocessor) and rig control circuits.
The "computer" and "radio" sides are isolated by optical isolators for
data circuits (FTDI FT232 to processor) or transformer (audio Line In
and Line Out).
73,
... Joe, W4TV
On 8/25/2016 8:41 PM, Peter Laws wrote:
Let me reiterate this again ...
I mean,
isolation transformers and relays and whatnot probably don't have a
lot of variation, but I would expect that there would be some
differences in chipsets.
I'm interested in an evaluation of the sound processor in the various
interfaces. Those can be evaluated without regard to people's tastes.
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