Digital Echo Canceller FAQs
- How to analyze captured files with the DEC application?
- How do I start recording simultaneously and stop recording simultaneously from multiple cards and timeslots?
- Can I use the Dual Laptop T1 or E1 Analyzer to run the DEC (xx066) application?
- What are typical delays in voice connections for different networks?
- Issue - "A recent wireless phone call I made had persistent echo."
- Issue - "Another had a consistent burst of echo each time I began to speak."
- Issue - "Yet another call suffered from high levels of background noise."
More to come soon…
How to analyze captured files with the DEC application?
Captured files can be analyzed directly with the DEC application, experiment with the sample data below
- css-s10 file test original.ula -- use this file for Rin Source - it is available in the DEC subdirectory
- css-s10 file test echo.ula -- use this file for Sin Source - it is available in the DEC subdirectory
- To get a feel for these files, view them with Adobe Audition in stereo mode - to do this use the "View PCM" application
under Call Capture and Analysis. Adobe Audition is required to view in stereo mode.
- Run the DEC with one timeslot selected. Leave the Rout Destination and Sout Destination unselected.
- The DEC will process the files to completion and provide the impulse response last calculated.
- If the Sout destination is selected as T1 Out, then the files are processed as if in realtime. The T1 Out selection forces
the processing to take place at the clock rate.
- If Sout destination is selected as File, then the output is a file, which can later be viewed with Adobe Audition.
How do I start recording simultaneously and stop recording simultaneously from multiple cards and timeslots?
Use "Record from Multiple Cards". This application is meant specifically for this purpose.
Can I use the Dual Laptop T1 or E1 Analyzer to run the DEC (xx066) application?
Yes, but you must limit the active timeslots (on the Dual Laptop) to just one or two and the DEC application to just
one or two as well. This is because the Dual Laptops have a thruput limitation
vis-à-vis the PCI Cards. A thorough
discussion of this is given in the Software Installation guide for Dual Laptops.
What are typical delays in voice connections for different networks?
The diagram tabulates delays in various networks.
Issue - "A recent wireless phone call I made had persistent echo."
If the echo is clean with little distortion, then it is likely that the echo path delay is beyond the EC's limit. If the echo
is distorted or partially cancelled, then its likely that the path delay and the EC limit are about equal and the dispersion
of the impulse response spans the EC's limit, resulting in partial echo cancellation. The
diagram shows that in some situations, the echo path tail can
be extended, resulting in the EC seeing a longer tail delay than it is capable of canceling. The DEC tool can readily detect
this problem (graphically and by tabular measurements).
Issue - "Another had a consistent burst of echo each time I began to speak."
The echo tail the EC sees should be constant, but if its not, the EC will constantly reconverge. During the time required
for the EC to fully reconverge (usually seconds), echo can be noticeably heard. Time varying echo paths are unusual, but in
todays world its possible for the tail network to encounter ATM or VoIP (see
diagram). Due to the buffering and delays in such networks, they
can be the cause of a time varying echo tail. The design of the network should be such that ECs see a constant tail delay
(during a connection).
Issue - "Yet another call suffered from high levels of background noise."
Mobile calls and the mobile environment can be the source of excessive background noise. Car, street, and crowd noise
are typical examples. These sources of noise can drown out the talker as well as making it difficult to hear the landline caller.
Noise and speech level measurement tools (like GL's Active Voice Level - see
screenshot) are crucial to quantifying the severity of the problem.
These days, ECs come with VED (Voice Enhancement Device) capability. For an introduction to VED, click on this
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