Nuts and Volts - September 2008

show the unmodified and modified cables.

Once we’ve got things connected, the rest comes pretty easy, though I did find a couple traps along the way. Let’s start with simple playback.

SUB VM_PLAY
  tmpW2

= __WPARAM12

TX_STR  “VPF “
TX_STR  tmpW2
TX_STR  “.MP3”
TX_BYTE CR
isPaused = No
ENDSUB

code, we’re sending numeric values to the player in indicated hex ($) mode; this is done using the TX_HEX²subroutine.

What happens if we want to move the sound from one side to the other? There is no easy command in the VMUSIC2 command set to do this, but there is an advanced command that lets us write to registers inside the MP3 decoder. We must be very careful when doing this — my VMUSIC2 made some really interesting and really awful noises when I started experimenting with direct register control.

SUB VM_PAN
  tmpB3 = __PARAM1
  tmpB4 = __PARAM2

With VM_PLAY, we send an inline file name (without the .mp3 extension) or a label that has a z-string with the file name. TX_STR and TX_BYTE take care of sending the name, extension, and a carriage return to the player. I’ve included a flag in the program that keeps track of the paused state of the device, so this gets cleared on a new play command. Keep in mind that if you send one play command while another file is playing, the original file will be stopped and the new one will play.

And speaking of stopping ... the VMUSIC2 player has just one processor that handles serial I/O with its host (the SX, in our case) and pulling data from the USB memory stick to send to the MP3 decoder. So, if you send any command when a file is playing this can interrupt the audio output. I bring this up because there are commands that we may want to send while a file is playing, specifically volume control.

tmpB3 = tmpB3 MAX $FE
tmpB4 = tmpB4 MAX $FE

TX_STR “VWR $0B”
TX_HEX2 tmpB4
TX_HEX2 tmpB3
TX_BYTE CR
ENDSUB

SUB VM_VOLUME
  tmpB3 = __PARAM1

tmpB3 = tmpB3 MAX $FE

TX_STR “VSV $”
TX_HEX2 tmpB3
TX_BYTE CR
ENDSUB

The VM_VOLUME subroutine takes care of setting the output volume of the VMUSIC2 player. It seems a little odd at first that we use zero for the loudest volume; what we’re actually setting inside the player is the attenuation from maximum loudness. Note, too, that the player sets the left and right channels to the same level. In order to simplify the SX

■ FIGURE 2. VMUSIC2 cable.

I’ve created a subroutine called VM_PAN that expects levels for the left and right channels to be passed to it, then writes those levels directly to the MP3 decoder chip. It does this with the VWR (write to register) command; the target is register $0B which is a 16-bit value holding the channel levels. The tricky bit with the VM_PAN routine is that its settings get wiped out by a play command — this is something that the VMUSIC2 firmware does.

So, if we want to play a file from only one side, for example, what we actually have to do is start the file and then immediately call the VM_PAN subroutine to move the audio as required. Unfortunately, if we do call VM_PAN in the middle of audio, there may be a little “click” in the audio output; this will depend on what’s in the MP3 decoder buffer — and this is where one could make an argument for a higher baud rate to minimize the communications time to the VMUSIC2.

Vinculum supplies a program called VncFwMod.exe that can be used to update the firmware file which is where we change the baud rate. This program is available on the Downloads page of the Vinculum site and is listed as Vinculum Firmware Customizer. I actually keep a copy of this on my memory stick so that I can plug the stick into my computer at any time and update the firmware file. It’s a good idea to use a different revision code when you change the firmware, and do remember that you need to cycle power to the VMUSIC2 for updated firmware to be loaded.