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Audio Codecs in Exchange Online

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Topic Last Modified: 2013-11-20

In Unified Messaging (UM), an audio codec is used to store voice mail messages. Unified Messaging can use any of the following four audio codecs to create and store voice messages:

  • MP3 (default)
  • Windows Media Audio (WMA)
  • Group System Mobile (GSM) 06.10
  • G.711 Pulse Code Modulation (PCM) Linear

The term codec is a combination of the words "coding" and "decoding" and is used with digital audio data. A codec is a software program that transforms digital data into an audio file format or audio streaming format. Codecs are used to convert an analog voice signal to a digital version of the voice signal. Codecs can vary in their sound quality, the bandwidth required to use them, and the system requirements needed to do the encoding.

Unified Messaging dial plans are integral to the operation of UM. By default, when you create a UM dial plan, the UM dial plan uses the MP3 audio codec to create and store voice messages. However, after you create the UM dial plan, you can configure it to use WMA, GSM 06.10, or G.711 PCM Linear audio codecs.

Each audio codec has advantages and disadvantages. The MP3 audio codec was selected as the default audio codec because of its sound quality and compression properties. GSM 06.10 and G.711 PCM Linear audio codecs were included as available options because of their ability to support other types of messaging systems.

When you plan for UM, you must balance the size and the relative quality of the audio file that will be created for voice messages. Generally, the higher the bit rate for an audio file, the higher the quality. You must also consider whether the audio file is compressed. The following table shows the sample bit rate (bit/sec) and compression properties for each audio codec used in UM.

Default UM voice message storage codecs

Voice message storage codec Bits Compressed file?

MP3

16 bit

Yes

WMA

16 bit

Yes

G.711 PCM

16 bit

No

GSM 06.10

8-bit

Yes

In UM, the file type created for a voice message depends on the audio codec that’s used to create the voice message audio file. The MP3 audio codec creates .mp3 audio files, the WMA audio codec creates .wma audio files, and the GSM 06.10 and G.711 PCM Linear audio codecs create .wav audio files. All these audio files are sent together with an email message to the recipient of the voice message.

Frequently, but not always, coding and decoding the digital data also involves compression or decompression. Audio compression is a form of data compression that reduces the size of audio data files. The audio compression algorithm used by the audio codec compresses the .wma or .wav audio files. In UM, the type of audio compression algorithm that is used is based on the type of audio codec selected in the UM dial plan properties. After the audio file is created and compressed, it's attached to the voice message.

Sometimes information from the digital data is lost during compression and decompression. The higher the compression that is used to compress the audio file, the greater the loss of information during the conversion. However, less disk space is used because the size of the audio file is reduced. Conversely, the lower the compression, the lower the loss of the information. However, more disk space must be used because of the increased size of each audio file.

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You can configure Unified Messaging to use one of the following four audio codecs for creating voice messages: MP3, WMA, GSM 06.10, and G.711 PCM Linear. By default, the MP3 format is selected.

The WMA audio codec is always stored in the Windows Media format, and the attachment is a file that has a .wma file name extension. Audio files encoded using the GSM or G.711 PCM Linear audio codecs are always stored in RIFF/WAV format, and the attachment is a file that has a .wav file name extension.

The size of UM voice messages depends on the size of the attachment that holds the voice data. In turn, the size of the attachment depends on the following factors:

  • The duration of the voice mail recording
  • The audio codec that’s used
  • The audio file storage format

The following figure shows how the size of the audio file depends on the duration of the voice mail recording for the three audio codecs that you can use in UM.

noteNote:
In this figure, the average length of a call-answered voice message is approximately 30 seconds.
UM_Message_Sizing

By default, the MP3 format is selected and is the default audio file format for voice mail messages. The MP3 format is a common audio file format that's used to greatly reduce the size of the audio file and is most commonly used by personal audio devices or MP3 players. MP3 is a cross-platform type of audio codec and is used for compatibility with many mobile phones and devices and different computer operating systems.

WMA is the most highly compressed audio codec of the three kinds of codecs. The compression is approximately 11,000 bytes for each 10 seconds of audio. However, the .wma file format has a much larger header section than the .wav file format. The .wma file header section is approximately 7 kilobytes (KB), whereas the header section for the .wav file is less than 100 bytes. Although WMA audio recordings are recorded for longer than 15 seconds, they become smaller than GSM audio recordings. Therefore, for the smallest but highest-quality audio files, use the WMA audio codec.

noteNote:
OWA for Devices can’t use the WMA format for voice mail. OWA for Devices only supports the MP3 file format.

The G.711 PCM Linear audio codec creates .wav audio files that are not compressed. Therefore, G.711 PCM Linear .wav audio files occupy the most space for any given duration when they're compared to the GSM and WMA audio codecs. G.711 PCM Linear .wav audio files occupy just over 160,000 bytes for each 10 seconds of audio. G.711 PCM Linear .wav audio files have the highest audio quality of the three audio codecs used by UM. However, the quality of comparable audio files created using the WMA and GSM audio codecs is acceptable to most users who listen to voice messages.

The GSM audio codec creates .wav audio files that are compressed. GSM .wav audio files are just over 16,000 bytes for each 10 seconds of audio. However, GSM creates an audio file larger than the audio file created by the WMA audio codec. Therefore, when you are balancing the quality of the voice message and the size, this may not be the best choice.

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