[Freeswitch-users] Proper prompt gain/level

[Jan Berger]

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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