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AAC Encoder on 5509A - 8000 sample rate input, beeps in output

James56380
Prodigy 70 points

Hello,

 

We're working on integrating the AAC library into one of our products, but many of the output files have several beeps in them.  Beeps tend to occur at the start of the file, and when a word is spoken after a short pause and are centered around 3200 Hz.  Some occurrences result in a series of beeps.  I am running CCSv3.3 with CGTv4.3.6 and AAC encoder 1.00.00.003 production.  This has been tested using the example TestAppEncoder on data with a sample rate of 8000.  I've tried all bit-rates from 8000 to 40000 with little luck.  A bit-rate of 8000 resulted in output without beeps, but the sound quality does not meet the needs of the project.  All other bit-rates yielded output that still contained beeps.  

 

I also tried hard-coding different sample rates into the example program (12000 and 16000), which resulted in files without beeps.  Of course at the higher sample rates the input is sped up and barely intelligible once encoded, but it leads me to suspect that there is an issue with encoding files with a sample rate 8000.

 

Another interesting data point is the output bit-rate.  Though I have specifically configured a bit-rate in the TestVecs.cfg file, the resulting file rarely matches that bit-rate exactly.  8 kHz input configured for 8 kbps results in 9 kbps output, and continues in a similar progression for each bit-rate configuration (one additional kbps for every eight).  So configuring for 40 kbps results in 45 kbps output.  I've tested various combinations of sample rate and bit-rate, and the only combination that resulted in the expected bit-rate was 16 kHz configured for 8 kbps.

 

I'd like to get this working with 8 kHz input at a bit-rate better than 8 kbps.  Is this possible?  What might be causing the beeps in the output?  Any suggestions would be helpful.

 

Thanks,

 

James

 

 

  • 0
    TI__Mastermind 30480 points

    Hi James,

    8kbps for a 8kHz mono signal is the minimum bit-rate we can achieve with AAC_LC Encoder. 

    Quality can not be guaranteed at this bit-rate. No encoder can encode at 8kbps with quality. You need to increase the bit-rate to either 16 or 24kbps.

    The variation in the bit-rate is because, the aacplayer is displaying average bit-rate for whole stream.

    Ram

  • 0 in reply to Ramprasad
    Prodigy 70 points

    Thanks Ram,

     

    I've tried encoding an 8 kHz sample rate input file at bit-rates of 16000, 24000, 32000, and 40000.  All bit-rate configurations (except 8000) result in output with random beeps.

     

    Do you know the source of these random beeps?  Is it even possible to encode 8 kHz input files with the AAC_LC encoder?

     

    James

  • 0 in reply to James56380
    TI__Mastermind 30480 points

    Hi James,

    I have also been using 1.00.00.003 AAC-LC encoder and could encode 8kHz input files. Can you please send your input file to me to x0038811@ti.com? I am observing quality improvements when I increase the bit-rate. There could be issue with memory placements.  You had some query on placements in external memory right few months back? 

    Ram

  • 0 in reply to Ramprasad
    Prodigy 70 points

     

    Ram,

     

    I've sent the original file and the encoded version.  The file has a sample rate of 8192, so you will need to configure the encoder for 8000. 

     

    Yes, I was testing external memory placements, but this testing was conducted independently.  I have setup my current tests using the original TI example code configuration to eliminate my custom memory placements as a source of error.

     

    James