[Freeswitch-users] Proper prompt gain/level

[Sidharth Kshatriya]

Yeah sox is great for command line and batch changes to volume levels

// this command invocation outputs the maximum volume increase
<name-of-input-sound-file> can go through without any distortion. The
maximum volume increase possible is stored as a decimal in max_vol.txt
sox <name-of-input-sound-file> -n stat -v 2> max_vol.txt
// This command actually boosts the volume level of the input file. Output
is stored in <name-of-output-sound-file>
sox -v `cat max_vol.txt` <name-of-input-sound-file>
<name-of-output-sound-file>

On Mon, Jun 27, 2011 at 4:34 AM, Nandy Dagondon <gcd at i.ph> wrote:

> 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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>
>
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-- 
Sidharth Kshatriya
www.sidk.info
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