Pulse Code Modulation is a digital encoding of an analog audio signal. PCM is the native audio encoding format for voice and other audio on wireline and mobile networks based on the TDM standard.

TelcoBridges and PCM

Applications on the Tmedia or Tdev platforms that require audio to be recorded or played back use TB StreamServer which provides native support for PCM encoded audio files.

An example of the use of TB StreamServer and PCM files is the following: two participants are on a conference call hosted by a Tmedia or Tdev device; the call is being recorded for posterity. The first person accesses the call via a traditional TDM-based landline telephone; the second accesses the call via a VoIP-enabled device. Since the voice media on the TDM landline is already encoded in PCM by the caller's handset, the audio is captured by the Tmedia or Tdev device and written to disk with no audio format conversion required. However, for the person using the VoIP device, there are a few extra steps involved. In this case, the Tmedia or Tdev device must first convert the call media from whichever voice encoding method was being used into PCM format. In short, the RTP packets are sent to the VoIP chip on the Tmedia or Tdev device from where they are converted to PCM and then written to disk in PCM format for storage.

When the contents of the conference call are played back for a called using VoIP, the process is reversed. The audio is taken from PCM format and converted into a codec that can be used over a packet network like the Internet. Should another person using a TDM-enabled landline also choose to playback the conference call, no conversion is required. The audio remains in PCM format throughout the process.