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Codec Setup

The codecs are quite complex, and can generally run in a large number of modes, though they do have a battery backup to save their configuration when powered down.

The software does not implement full H221 so it is vital that the codec is in the correct configuration. This is set up using the RS232 control port from the workstation. We've experimented with automatic setup of the codec, but with limited success, so it's more certain if performed manually. At UCL, we simply use ``tip" to connect to the codecs as we wanted the form of output to be as simple as possible, but the codec supports more complex terminal emulation, so tools such as ``kermit" can also be used. The correct RS232 parameters for the various codecs are (in /etc/remote format assuming connection to the sun's RS232 port A):

gpt_system261:dv=/dev/ttya:br#9600:el=^C^S^Q^U^D:ie=%$:oe=^D:pa=odd:
bt_vc2100:dv=/dev/ttya:br#9600:el=^C^S^Q^U^D:ie=%$:oe=^D:pa=odd:
bt_vc2300:dv=/dev/ttya:br#2400:el=^C^S^Q^U^D:ie=%$:oe=^D:pa=odd:

If you have got this correct you should see when you power up the codec a welcome message from the codec, and on hitting return, a status report:

System 261 Video Codec - Status Codec Location = User = Video Transmit = Face-to-Face CIF 30 Frames/Second Video Receive = Face-to-Face CIF Full quality Video processing now selected by the user Audio Transmit = none Muted Audio Receive = none Muted Channel Interface = RS449 at 128kHz 64kBit Timeslots Data Transmit = 128kBit/s (2 x 64k) Requested = 128k Video Data=126.4k Data Receive = 128kBit/s (2 x 64k) Video Data=126.4k

X21 Data Ports - A Transmit = Off A Receive = Off B = Off Audio = Off No errors in decoder Channel decoding No errors in encoder video input Enter a command, or HELP for command instructions

This report is from a codec that is correctly configured, and is sending and receiving from the RS449 interface at 128Kb/s.

To get the codec towards this state, a number of commands should be issued. (You should only need to do these when the codec has been previously configured for some other use, or when new - you shouldn't need to do them all every time!)

Each command should be acknowledged by the codec with an ``OK". If instead it gives an error report, you'll have to resort to the codec manual.

- tells the codec to use the RS449 interface.
- switches encoding of audio off
- tells the codec to treat multiple 64K channels as one larger pipe.
- 56K mode is used for US ISDN and leased lines - we want nothing to do with it.
- sets the clock rate the codec is expecting to see on the RS449 to 128Kb/s
- sets the transmission rate to 128Kb/s
- cancels previous frame rate reduction commands
- cancels various quality reduction commands
- turns off the external X21 data port A
- sets the RS449 clock source

After doing all this, typing should result in:

System 261 Video Codec - Status Codec Location = User = Video Transmit = Face-to-Face CIF 30 Frames/Second Video Receive = none Full quality Video processing now selected by the user Audio Transmit = A-Law 56kBit/s Muted Audio Receive = none Muted Channel Interface = RS449 at 128kHz 64kBit Timeslots Data Transmit = 64kBit/s (1 x 64k) Requested = 128k Video Data=6.4k Data Receive = 128kBit/s (2 x 64k) Video Data=126.4k

X21 Data Ports - A Transmit = Off A Receive = Off B = Off Audio = Off Warning - RS449 Channel decoder RT clock input signal missing No errors in encoder video input Enter a command, or HELP for command instructions

If for some reason, you don't have something similar, you should probably try to figure out why at this stage, before proceeding.

If you get this, then issue the command . This will put the codec in a loopback mode that will check the parameters you've just set. You should get the camera's image fed back to the monitor via a 128K data stream, and get a moderate quality image slightly delayed. The command should now result in:

Codec Location = User = Video Transmit = Face-to-Face CIF 30 Frames/Second Video Receive = Face-to-Face CIF Frozen Full quality Video processing now selected by the user Audio Transmit = none Muted Audio Receive = none Muted Channel Interface = RS449 at 128kHz Codec Loopback On 64kBit Timeslots Data Transmit = 128kBit/s (2 x 64k) Requested = 128k Video Data=126.4k Data Receive = 128kBit/s (2 x 64k) Video Data=126.4k

X21 Data Ports - A Transmit = Off A Receive = Off B = Off Audio = Off No errors in decoder Channel decoding No errors in encoder video input Enter a command, or HELP for command instructions

Important here is that transmit and receive video data rates state 126.4K If they say 124.8K, you should issue the command . If they say anything else, something is not configured correctly, and again you'll have to refer to the manual.

Also important is that the display states . If it says anything else, issue the command .

Now issue the command to continue.

The line means the codec is not seeing a clock from the HSI board. At this point you should load the UCL GPT/HSI device driver, and set the device driver data rate to 128K. How to do this is described elsewhere.

Once the driver is loaded and set to 128K, the line

should change to

This means the codec is now receiving a clock, but is not receiving any valid H221 data.

If you do not see this message, it probably means that either the device driver didn't load correctly, or you have a faulty RS449 cable. You should sort out either problem before continuing.

When you get the message , the codec is ready to talk to the workstation. As a test that this link is working correctly, issue the command to a shell on the workstation. This will use the dd program to grab data from the codec, and loop it straight back out to the codec. The codec window should now give the following in response to the command:

X21 Data Ports - A Transmit = Off A Receive = Off B = Off Audio = Off No errors in decoder Channel decoding No errors in encoder video input Enter a command, or HELP for command instructions

You should also see similar quality looped back video to that obtained using , but with a slightly larger delay as it's now traveling twice through the device driver in the workstation.

At this stage, codec configuration is complete, except for changes of data rate, frame rate and image format that you might want to make later. Stop the dd program. Useful codec control commands are listed in appendix 1.

mhandley@cs.ucl.ac.uk
Thu Dec 30 15:40:00 GMT 1993