U.S.Robotics Sportster Voice

U.S.Robotics Sportster Voice

This note applies to Texas Instruments-based modems as well.

The program was initially developed for these modems, it is fine-tuned and optimized for them.

Modem selection

There are currently three subsets of U.S.Robotics voice modems supported: modems with IMA ADPCM voice codec (U.S.Robotics / 3COM Sportster Voice), which work best, a modem with an un-encapsulated GSM codec (Sportster Voice 33.6/56K), a modem with an encapsulated GSM codec (Sportster Voice 56k X2) and older Sportster Vi. If one you have selected does not work, try another one.

Available Voice Codecs
Method Bit Depth Sampling Frequencies Comment
IMA ADPCM (U.S.Robotics) 4 8000 Good sound quality (read 'Known issues')
GSM 6.10 - 8000 Considerable good sound quality
Voice compression methods

Modern U.S.Robotics modems all come with a built-in IMA ADPCM voice codec. This type of voice compression gives the best available results.

Set GSM 6.10 voice compression for Sportster Voice modems as it was initially implemented in the program and widely tested. One difficulty with this type of compression: some modems do require GSM encapsulation and some don't. Usually this is auto-configured, but sometimes you may want to change it.

With the release of x2-based modems, the option to enable UN-encapsulated GSM is available. The S41 (Voice Options Bitmap) register defines the method. During playback to the modem, it will always auto-detect encapsulated GSM, un-encapsulated GSM or ADPCM data. During record from the modem, S41 will determine the format of the GSM data if GSM was selected as the audio data format.

My G.721 ADPCM (4 bps) implementation does not seem to work with this type of modem. ADPCM-compressed files play right, but received data sounds garbled.

Known issues

Some modems do not report silence when using IMA ADPCM voice codec. The driver 'U.S.Robotics / 3COM Sportster Voice' uses IMA ADPCM. If silence detection is critical for you, you should use either 'U.S.Robotics 33.6/56K Voice' or 'U.S.Robotics 56K Voice X2'.

Modems with GSM 6.10 only

Sportster models differ a bit from each other in handling GSM-compressed data. Vi modems require encapsulated GSM frames. Older 33.6k modems require un-encapsulated GSM and do not 'understand' encapsulation. Newer 33.6 and all 56K models can work with either encapsulated or un-encapsulated GSM frames.

Select Voice 33.6/56k modem for un-encapsulated GSM. You should choose Voice 56K X2 or Vi modem for compatibility only if program does not handle GSM voice data correctly.

Modem Notes Modem Notes