May 26, 2004
Note: This Field Notice is a legacy Latitude Field Notice that has been converted to the Cisco format so the information would remain available to their customers.
On a 5.0.0af system, when the configured echo canceller tail length configport parameter 15 was changed from the factory default 64 ms to the max 128 ms value, echoes with tails or round trip delays longer than 64 ms were not cancelled adequately. In addition, these long echos resulted in other voice quality issues such as choppy voice and sudden variations in loudness. Restoring the configured echo tail back to 64 ms eliminated the choppy voice and echo canceller operation returned to normal.
The Voice Resource Management (VRM) digital signal processor (DSP) uses third party echo canceller firmware that is initialized once at boot time by running an initialization function. This function allocates the maximum memory recognized by the echo canceller firmware according to a maximum echo tail length passed parameter. This passed parameter is currently hard-coded to a 64 ms maximum value. If this parameter is less than 128 ms, then the echo canceller is not using all the 128 ms worth of data memory set aside in the hardware memory map for its adaptive filters for each channel, also known as port. The configport echo tail parameter 15 may be updated during run time to values greater than the initialized tail, and this is where problems are likely. In addition, changing the configport echo tail then restarting the system still results in these problems. Since the echo canceller's voice channels are adjacent to each other in DSP memory, data overlaps and corruption are possible if the configured value 128 ms is larger than the hard-coded initialization value 64 ms.
In the current M3 architecture, there are 8 channels of echo cancellers running simultaneously on a single VRM DSP.
Echoes in the range of 64 to 128 ms are not adequately cancelled on R5 servers. This is true even when the configport echo tail length is extended to 128 ms. This uncancelled echo may be heard as buzzing or muffled echo, but it appears that mis-configured headset amplifiers are mostly responsible for this type of echo. More likely, the data overlaps mentioned above and the resulting data corruption will be heard as choppy voice and sudden volume changes.
Voice with echoes shorter than 64 ms will be cancelled normally, and do not exhibit voice quality issues due to uncancelled echo, choppy speech or sudden volume changes.
This issue does not affect R4.3 and PCI platform servers.
A new VRM DSP firmware image (vrmdsp126.96.36.199.bin) has been created that initializes the echo canceller to the maximum per channel memory allocation of 128 ms, the maximum allowed by the in the DSP memory map per channel. This image must be loaded on the M3 server, copied into the vrm3dsp.bin file, and the server restarted. The configport echo control value may then be increased up to the maximum of 128 ms.
A hot fix for 5.0.0af is available; contact Cisco TAC if you need to have this applied. This will be incorporated into the next release.
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