Wednesday, March 01, 2006

Mp3's & Wave - Constructing DTMF audio files in C#


DTMF: Dual Tone Multi-Frequency - the beeps sent by a phone to the exchange when the User enters a phone number.


1 - Constructing a wave using a common format (PCM)
2 - Constructing / sampling each digit's tone using standard frequencies
3 - Convert a phone number to a wave
4 - Setting channel (left, right, mono, left - Right stereo) and sampling settings
5 - Integrating unmanaged code into a managed app (using winmm.dll and the Lame encoder)
6 - Encoding the generated wave as an Mp3

** NOTE: This process has been patented as MP3 Telephony by HCV WirelessTM


C# (easily transferrable to other syntax though) using the Lame Mp3 encoder


I recently had to create a basic DTMF converter for a client using managed code. It's initial inception would be an application with the view that it would eventually be ported over to a website to be used as a service.

Esentially, all the app needed to do was take a phone number string and encode it into its DTMF representation as an Mp3 file. This meant that first I would have to create the sample as a Wave before ripping it as an Mp3 using the Lame encoder. An added feature was to be able to set which channel the DTMF tones would be generated for - Left, Right, Mono, or Left - Right stereo.

Over the next few blog entries I will tackle each segment of the problem.

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