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  • AGC?

    I'm having a weird issue with the 14D...when I play any of my messages, the volume starts at one level, then gradually rises until the message is either distorted, or almost distorted. It almost sounds like automatic gain control "overzealously" compensating for low volume recordings. The original wave files, which are professional studio recordings, do peak at close to 100% volume without distorting in any way.

    The distortion is present on both the speaker output and the direct audio output of the 14D. Both the original .wav files and the decoded .ad4 files encoded from them sound fine.

    All these messages are relatively short (less than 4 seconds). For these short messages, I obviously need the volume to start out at the proper level from the very beginning...and, of course, the distortion is unacceptable. Any thoughts?


  • #2

    Don't think there's any AGC there

    How are you controlling the SOMO?

    Are you controlling it serially and perhaps your code is sending volume up?

    Have you added a homebrew external amplifier that is somehow increasing it's gain oevr time?


    • #3

      Thanks for the reply. It's being controlled serially, so I'll check the code again. This design is a replacement for an obsolete ISD chip and does have an external amplifier, but the amp stays on all the time and is definitely stable. The amp is driven from the speaker output of the 14D via a small audio transformer.

      I also observed the same phenomenon at the single-ended audio output (pin 14) using an audio tracer, so it doesn't appear to be the amplifier or something more esoteric like transformer overload.


      • #4

        Here's an interesting observation. All of the files I'm using were professionally recorded and, as mentioned earlier, were volume normalized to 99% on peaks...and all of them exhibit the phenomenon I mentioned in my first post. After playing back the sample files and discovering that they played perfectly, I took my audio files and reduced their maximum peaks from 99% to 75%, then re-encoded. No "volume creep" or distortion was noted on the modified files! Very strange...can the 14D receive embedded commands from its external memory?

        If anyone is interested in replicating the phenomenon, I'd be glad to PM a copy of one of the files that is giving me trouble.