[Freeswitch-users] Proper prompt gain/level

[Nandy Dagondon]

you can also run sox on a script to change the volume of the sound files by
batch.

On Mon, Jun 27, 2011 at 5:20 AM, Jan Berger <jan.berger at video24.no> wrote:

> Wavepad is free and will do bulk changes to sound files.
>
> Won't AGC solve this?
>
> Jan
>
> -----Original Message-----
> From: freeswitch-users-bounces at lists.freeswitch.org
> [mailto:freeswitch-users-bounces at lists.freeswitch.org] On Behalf Of Bryan
> Smart
> Sent: 26. juni 2011 22:32
> To: freeswitch-users at lists.freeswitch.org
> Subject: [Freeswitch-users] Proper prompt gain/level
>
> As part of creating prompts for my IVRs, I've tried to match the audio gain
> of my prompts with the gain of the stock English prompts. During this
> process, I noticed that the English Callie prompts are recorded extremely
> low (max gain around -16DB). I have the Cepstral Callie voice, and I must
> set the Cepstral volume to about 50% in order to match the gain of the
> English prompts. In conferences and other situations where prompts are
> played over conversations, the level of the prompts are obviously low.
>
> I can, of course, renormalize the gain of the prompts up to -10DB or more
> with a sound editor. However, I wonder if there is a better way to change
> the level of the prompts, or if there is a good reason for the prompts to
> be
> encoded at such a low level.
>
> I haven't considered all of the implications yet, but I'm fairly sure that
> encoding the prompts this quietly is not the best approach, even if it is
> desirable for the prompts to play quietly on a call. For each 6DB reduction
> in gain, there is a 50% reduction in perceived volume, and one less
> significant bit is used for storing the audio. In a 16-bit file, a maximum
> gain of -16 means that only the 14 least significant bits are actually used
> for encoding the audio. This results in a reduction in dynamic range, but
> the difference isn't really noticeable as long as the data remains 16-bit.
> The problem comes when the audio is converted to a different bit depth. For
> example, most quickie routines for converting 16-bit audio to 8-bit audio
> will simply chop off the 8 least significant bits. Therefore, when the
> prompts are converted to 8-bit audio for use by most of the narrow band
> codecs, the prompts are only using 6 bits of audio. If the volume of the
> channel is increased, then the
>  6 bits are promoted, and the dithering errors at the bottom become louder.
> In the worst case, since these prompts are only encoded with 14 bits of
> actual data, and converting to an 8-bit channel will only leave 6 bits of
> actual data, boosting the gain of the prompts on an 8-bit channel to full
> loudness would result in the noise floor (the level of the dithering
> crackle) being about -18DB. That's almost as loud as the prompts themselves
> sound at the moment.
>
> Anyway, regardless of observations, is there a reason why the prompts must
> be recorded so quietly? If I'd like them louder (without increasing the
> gain
> of the entire channel), is there a way other than running them through a
> tool to renormalize their max levels?
>
> Bryan
>
>
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