Processing of the core/extra sounds

As we know, there are quite a few soundprompt packs (and even some music tracks) available either through the installer or for download under Index of /pub/telephony/sounds/releases/ . However, these sounds are all quite grainy (even the .wav format versions), and have been fed through a 3kHz low-pass filter.
In order to create my own soundprompts in the the same style as those provided,

  1. Are the original, unprocessed recordings available anywhere?
  2. Is the method used to process these recordings (and convert them into the different formats as well) known?

The .wav format ones have to be passed through a 3.4kHz low pass filter, as they are sampled at 8kHz, and to achieve reasonable anti-aliasing you need to use a filter significantly below the Nyquist frequency of 4kHz. Standard phone audio is 300Hz to 3.4KHz.

The format one would expect to have the highest quality is .sln16, which is sampled at 16kHz. It looks to me as though the sln16 files have frequencies up to 7kHz.

g.722 will also give you 7kHz audio, but with a higher signal to noise ratio.

Not much of the phone system is capable of much better than G.722, and a significant part only supports 3.1kHz audio. Mobile phones generally use speech only codecs, although ones covering 7kHz are becoming more common.

I think you would probably have to contract directly with Alison for the master recordings.

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