David/All,
I’ll answer this on a separate thread cause it gets too confusing when mixed in
with the other comments.
In .NET I have used Microsoft’s Direct X. Direct X is a large (and cumbersome)
beast is you use all its capabilities , graphics, animations, sounds, etc but
what is normally done for sound cards is just use a component of Direct X
called Direct Sound. This is much smaller and manageable and I have had good
luck with it over the years. On the down side there isn’t (or at least wasn’t
last time I checked a few years ago) much help in the way of descriptions and
examples. I had to plow through some obscure documents and do a lot of trial
and error and get a some clarifications from some of the Microsoft forums.
Basically this is what you need to be able to do with a sound card for DSP work
like you would encounter in CODEC2 or the FDMDV modem:
1) Have the OS be able to enumerate the installed sound cards and how to
describe and identify them. Sounds easy enough but...
a) Some sound cards have different names for the capture and
playback....those are actually two different devices in Windows.
b) You have to make sure you can differentiate between two identical
installed sound cards...e.g. two or more SignaLink USBs plugged in. It isn’t
that rare and some chip sets report the same “friendly name” to the OS. I
ended up tagging on some unique characters extracted from the driver GUID to
make otherwise identical names appear different to the user.
2) You have to be able to set the parameters desired for the sound card. In
Direct X these include:
For capture devices:
Buffer size
Sample size in bytes (most sound cards are 16 bit ...some are now 20,
22, or 24 bits)
Sample rate (samples per second) best to generally stick with the
cardinal rates of 48000, 96000, 192000 and decimate from there if needed.
Mono or Stereo. (Stereo not supported on most ham radios of course)
Notify size: this tells the driver what size chunks to interrupt the
program with (usually in a separate “acquisition thread”) . Notify size is
usually set to a binary value of samples. e.g. 1024, 4096 etc. There is no
absolute guarantee you will always get this number of samples ...a higher
priority task may interrupt and you could get an integer multiple of the Notify
size. In any case you shouldn’t ever miss any samples
For Playback devices similar parameters but no notify size. In every sound
card I have seen you can treat playback and capture as two different devices.
3) You may also have to interact with the Windows mixer that controls the
volume etc. Some sound card drivers use the mixer...some do not. I prefer not
to use the mixer if possible...little things like a Windows auto update may
reset some volume levels for instance!
4) Playing with the Playback device. There are basically two methods both are
useful.
a) Play a file: You create a file (usually a standard formatted .wav file
the file header specifies the sample rate, bytes/sample, mono stereo etc.) You
tell the Playback device in Direct X to play the file. You can abort the play,
check on status (e.g. is it actually playing now or not) etc.
b) Play a stream. You can define a stream in Direct X which has basically
all the characteristics of a file but it is a dynamic buffer (FIFO). You can
append samples to the stream before or while it is playing. Once samples are
played the by the playback device they are purged from the buffer. This has
two major advantages:
1) It doesn’t use disk space or require disk access but an efficient
thread-safe buffer manager
2) You can continuously stream data ...very important for something
like continuous voice. When the stream ends (runs out of data) the playback
device automatically goes back to the not playing status.
Although I originally used a) above for the last couple of years all
the WINMOR and V4 work use b) even though they are not continuous (sending
typically 3- 6 second “frames”) . The streams just have less overhead and work
well as long as you don’t try and feed in samples a lot faster than you are
playing of course!
Basically you need about half a dozen fairly simple .NET utility functions to
control the above. I would be willing to build a little demo application that
demos all the above.
There are some other drivers for sound cards on Windows but when you stick with
Direct X any device plugged in generally works automatically with either the
generic driver or Windows finds the required driver. For example plug in a
Icom IC-7200 into the USB port and there is a USB sound card (24 bits in this
case if needed) that can be enumerated and looks and works just like any other
Windows sound card.
So as an application a VOX (or PTT) operated application would need to:
1) Allow the user to select the voice input capture device. This would be a
microphone plugged into the computers sound card or more conveniently a
wireless head phone/mic that appears as a USB type capture device.
2) Allow the user to select the received and decoded voice playback device.
Again either the computers speakers or the earphones of the wireless headphones
above.
3) Allow the user to select the playback device to the radio. This could be a
generic sound card in the computer, a device like a SignaLink USB with an
isolated interface or the built in sound card of a radio like the IC-7200 or
TS-590S.
4) Allow the user to select the capture device from the radio audio. Either the
speaker, AUX audio out or in the case of the new radios the actual built in
sound card in the radio.
5) Then the app would have to:
For transmit:
Stream the microphone sound samples into the CODEC2 encoder
Stream the output of the CODEC2 in the FDMDV modulator
Stream the output of the FDMDV modulator into the Radio playback device
Activate the PTT if required.
For Receive:
Stream the audio from the radio to the FDMDV demodulator
Stream the demodulator output to the CODEC2 decoder
Stream the output of the CODEC2 decoder to the Speaker or headphone
playback device.
Although this all requires continuous streaming it’s a lot easier than an ARQ
type system with constant Data/Command and ACK/NAK cycles.
Over the next week I’ll try and cut and paste up some VB.NET (C isn’t that
much different in this case) examples with comments on the half dozen or so
functions needed.
73,
Rick Muething, KN6KB
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