Overview
The CMedia sound chip is an
inexpensive but generally capable chip found in sound
cards as low as $10. It is also found on some
motherboards. As well as the normal features, its
major selling point is the inclusion of SPDIF
(Sony/Philips Digital Interface, also "S/PDIF")
digital input and output (except for the
CMI8738/PCI-6CH-LX version and a 4 channel version
CMedia's website only hints at).
As I didn't look for SPDIF as a
feature when I bought my computer, it was indeed a
pleasant surprise to discover this capability lurking
within. I bought a Optimus DCC (Digital Compact
Cassette) deck back in December '95 when they were
closing them out. As well as having a few DCCs
around to transfer to computer or CD-R, this unit has
very good quality analog-to-digital converters -- I hear
no loss at all in a single pass. Use as an ADC is
even suggested in the manual... even a mechanically
worn-down deck could be used in this manner. To use
the unit as a converter, it is switched to analog input
mode and record is engaged.
If you need to get analog input at
good quality into your computer, this combination of
external conversion and SPDIF is a good solution if you
own or are considering buying a device capable of the
conversion: DAT, MiniDisc, DCC, or a standalone
converter. The converters tend to be at least as
good of quality as the converters in all but the best of
PC sound cards. The DACs also may be better.
There are other options. One is a high quality
sound card. This could include multichannel setups
made for musicians and is a more flexible option.
The other option is to use a standalone audio CD
recorder, and then "rip" the CD into the
computer.
I'll mention some other features of
the CMedia chips: full duplex operation, multichannel
capability, wavetable synthesis. The ADCs are
"voice quality" at best. The DACs have
some loss in the frequency extremes and a less than ideal
noise level but sound very good in normal
listening. A particularly important feature is that
the record side mixer is fully digital. Notably,
you can record from any WAV output device
digitally. This allows things like recording from
software synthesizers lacking built-in record capability,
random recording of streaming audio radio, and using
applications like Winamp as an EQ/effects unit.
CMedia provides no fancy software
extras. If you need a good recorder software,
you'll need to get it elsewhere (Windows' standard
recorder works, but is too limited). CMedia's
AudioRack application does provide normal players, and,
very importantly, the Mixer part of it provides SPDIF
control. You'll want this; it should come with a
card or motherboard, and the latest version is available
through their web site. I've had no problems in Win98, but
you might want to check for info on recent OSs.
Note, engaging SPDIF-in disables all normal recording
controls. If you need a good recorder software,
you'll need to get it elsewhere.
SPDIF details
Connectors vary on different
cards. There is even optical connectors on
some. On my motherboard, there is a 2x4 pin
header. CMedia shows a couple of "reference
designs" on their website which may help you figure
out the pinout, which could also use 2 pin connectors or
RCA phono jacks. One $10 card I saw had a space on
the board for a header, but it wasn't populated.
This could be added, although I didn't check to see if
any other components were missing.
You'll need an adapter to connect
to headers. I took a spare CD-ROM audio cable and
cut off one end and attached a phono jack. Since
these use a 3-pin connector, I had to swap around the pin
sockets so the unused pin didn't run into an
obstruction. Use the cable's shield for the ground
side.
SPDIF-in is usually
"terminated" as a 75 ohm load. CMedia
doesn't do this. As the normal consumer level SPDIF
is only 0.5 volts peak-to-peak, the resulting overshoot
desirably increases the available signal level up to
double. This can vary with different driving
devices and cables. With my DCC, it didn't work,
but try it before taking additional steps. The
SPDIF-out works fine.
The AudioRack mixer includes
options for the signal levels, inversion, 44.1/48 kHz
sample rate, and SPDIF-out and loop-through enables for
the CMI8738 -- earlier versions use a jumper for levels
and inversion. The level should be set properly for
output, but I'm not convinced the settings do anything on
the input side. I think it just auto-detects -- so
it may take restarting the SPDIF stream to get it to lock
on to the signal. The inversion feature is unusual,
since SPDIF is designed to be insensitive to
inversion. This is more to help it adapt to
different signal characteristics.
The SPDIF-in fix
As I mentioned, SPDIF-in didn't
work with my DCC. Tests have shown that only about
half of all consumer devices work with it. 'Net
wisdom is that the best way to deal with this is build an
external 0.5 to 5 volt converter circuit (several can be
found here). I tried this, but it didn't work,
probably due to parts substitution. I considered
there had to be a better way.
The CMedia circuit brings SPDIF in
as follows: first the signal goes through a capacitor,
for DC decoupling. This goes to the pin of the
chip. It also goes to biasing circuit consisting of
two 4.7K (kilo-ohm) resistors, one to ground and one to 5
volts (typical digital supply voltage). The
resulting bias is 2.5 volts. This appears to not
match the switching threshold of the input (which could
also vary in chip production).
One modification is to simply
reduce the bias by reducing the ground-side resistor to
3.3K or so. Since regular leaded resistors are
easier to get than the surface-mount ones in computers,
it is easier to add a 10K resistor in parallel.
This drops the bias by about 0.5 volts -- remember the
unterminated voltage swing is just short of +,- 0.5
volt. The bias could also be made adjustable,
although I tried this and found no change over the range
where it works.
UPDATE: Think
Multimedia card. Worked inititially, later stopped. Found
while board had 2-pin header labelled SPDIF-IN, this had
NO bias circuitry at all! Apparently my deck has an
output cap and the bias just worked ... Added input
capacitor (.1 uF), and ended up using bias of 10K
pulldown and 12.2K up (hardest part was finding a good
way to get 5V). This board is old but not as old as the
next one ... (end update)
Specific instructions:
Elitegroup P6SET-ML motherboard (also OEMed as simply
"Mainboard", for example in Powerspec 4312)
**NOTE before proceeding: I assume
no responsibility for anything bad that might happen if
you attempting something and it causes a problem. I only
certify I did this, and it worked.**
Notes on soldering to a
surface-mount resistor: the solder should get hot enough
to get smooth and reflective, but no hotter. You
can heat the whole resistor up so much that the solder on
the other end melts and it slides out of place.
After soldering one end, allow to cool before soldering
the other end. If you couldn't hold the resistor
perfectly still on the first connection, use that to help
hold it when doing the second and then redo the first.
These instructions assume the board
is right-side up, as it would be in a tower-style case,
with the chip writing also right-side up. Find the
CMedia chip on the board.
The SPDIF header is JP3.
SPDIF-OUT is the two pins on the far right, with signal
on the bottom (pin 7) and ground on the top (pin
8). SPDIF-IN is the two pins second to the far
right, with signal on the bottom (pin 5) and ground on
the top (pin 6).
There is a row of tiny components
above the CMedia chip -- surface mount resistors and
capacitors. The one you want is on the far left and
should say "472" on it in really small
letters. The modification is to add a 10K resistor
in parallel with this.
With my DCC, sometimes it locks on
first try, sometime it takes several retries. Retry
is by switching in and out of record mode or switching
off and on the DCC. Poor lock (sync/detect) is
apparent in a noise signal with barely audible
music/audio from the source. Complete lack of
detect is apparent by silence.
More information:
SAQ (Sometimes Asked Questions)
CMI833x Mailing List
Richard
Hanson's Digital Audio
CMedia
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