Is there an option in the BCM200 to adjust the RTD (Round trip delay)? We are getting a bad echo on external calls using VOIP phones over copper lines. You can hear yourself repeat what you say for about 10-15 sec than it goes away. Am I looking into the right setting or is there another setting I should be looking at? We have changed codecs and we hear the difference in the codecs but it still doesnt solve the problem.
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BCM200 RTD 1
- Thread starter DMilden
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CBasicAsslember
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The delay in a network is called latency. There is no setting for adding latency, nor would it make any sense to add latency. There is one thing you could try, although it may provide marginal improvement, is to use the highest bit rate codec. The reason this may help is as the compression ratio increases so does the number of bytes required to do the compression calculation. This of course means you have to have more bandwidth available. Traditional TDM uses a sampling rate of 8k bytes/sec, or put another way, a new sample is outputted every 125 microseconds. If the compression algorithm requires 100 bytes to do its calculation then the algorithm would add 12.5 milliseconds of latency. The latency is due to the fact that there can be no output from the algorithm until it has received 100 bytes. On the receiving end decompressing the data would also require another 12.5 milliseconds since it inverses the compression algorithm. The switches and routers that the data has to go through to get from the phone to the BCM will add even more latency. The total latency will be the time delay from end to end. The time delay for the echo will be twice the end to end latency.
So why is latency so important? It has to do with the way our brain interprets sound. Short delays (under 20 or 25 milliseconds) our brain interprets as being the same signal. As the delay time increases, dependent on the level of the delayed signal, we start to hear an echo. So where does the delayed signal come from? It is due to your voice leaking from the far end receive side to their transmit side. Your voice is now sent back as part of the far ends transmit side. This leakage is completely out of your control. In an attempt to fix this an echo canceller is implemented. An echo canceller attempts to create a signal (using your transmit signal) equal to the delayed signal in magnitude but opposite in sign. The 10 to 15 seconds it takes for the echo to go away is the time it takes the echo canceller to train itself.
So why is latency so important? It has to do with the way our brain interprets sound. Short delays (under 20 or 25 milliseconds) our brain interprets as being the same signal. As the delay time increases, dependent on the level of the delayed signal, we start to hear an echo. So where does the delayed signal come from? It is due to your voice leaking from the far end receive side to their transmit side. Your voice is now sent back as part of the far ends transmit side. This leakage is completely out of your control. In an attempt to fix this an echo canceller is implemented. An echo canceller attempts to create a signal (using your transmit signal) equal to the delayed signal in magnitude but opposite in sign. The 10 to 15 seconds it takes for the echo to go away is the time it takes the echo canceller to train itself.
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