Plextor ConvertX PX-M402U - Page 17

Audio conversion

The M402U can capture audio at the same time that video is captured, and it synchronizes these two streams to produce a single video/audio file.

When MPEG-2 and MPEG-1 profiles were used, we could convert audio into normal MP2 audio, into Linear PCM audio (similar to the Audio CD format), and Dolby Digital audio. Dolby conversion (AC3 file) is not hardware based but software based. Since the ConvertX can receive simple stereo audio channel data, it cannot in anyway convert audio to 5.1 channel. Even if the software could allow it, the produced audio stream would not be 5.1 channel data.

When MPEG-4 and DivX profiles were used, we could convert audio into MP2 or MP3 format. We used all available audio formats to investigate changes in CPU usage. Only when Dolby Digital encoding was used, the CPU usage was slightly increased. This occured because Dolby Digital encoding forces the software to encode the audio using a more advanced algorithm that requires more CPU power.

The following table shows all supported audio formats, along with audio sampling and bitrate.

Dolby Digital is encoded at 384kbps. We were impressed with this audio format. High sound clarity. There was a big difference between this audio format and the other formats. Vocal frequencies were never this clear. No loss at all in the midrange, and bass frequencies were deep enough, even for small audio systems.

Using the mp3 and mp2 formats, we heard losses in the midrange, but we were happy to see that bass frequencies remained deep. However, there is not much we can expect from these audio formats.

Linear PCM is encoded at 16bits/48kHz. This was the best format for audio conversion, even better than Dolby Digital. Dolby is encoded at 384kbps, while Linear PCM is encoded at 1536kbps. Of course, this difference produces bigger audio files, resulting in a bigger video/audio file.