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SPX Audio File Format Description:
Unlike many other speech codecs, Speex is not targeted at cellular telephony but rather at Voice over IP (VoIP) and file-based compression. The design goals have been to make a codec that would be optimized for high quality speech and low bit rate. To achieve this the codec uses multiple bit rates, and supports ultra-wideband (32 kHz sampling rate), wideband (16 kHz sampling rate) and narrowband (telephone quality, 8 kHz sampling rate). Designing for Voice over IP (VoIP) instead of cell phone use means that Speex must be robust to lost packets, but not to corrupted ones since the User Datagram Protocol (UDP) ensures that packets either arrive unaltered or don't arrive. All this led to the choice of Code Excited Linear Prediction (CELP) as the encoding technique to use for Speex. One of the main reasons is that CELP has long proven that it could do the job and scale well to both low bit rates (as evidenced by DoD CELP @ 4.8 kbit/s) and high bit rates (as with G.728 @ 16 kbit/s). The main characteristics can be summarized as follows:
Free software/open-source, patent and royalty-free.
Integration of narrowband and wideband in the same bit-stream.
Wide range of bit rates available (from 2 kbit/s to 44 kbit/s).
Dynamic bit rate switching and Variable bit-rate (VBR).
Voice Activity Detection (VAD, integrated with VBR).
Variable complexity.
Ultra-wideband mode at 32 kHz (up to 48 kHz).
Intensity stereo encoding option.
Audio Transcoder supports the following conversions with Speex format files:
• SPX to AAC
• SPX to M4A
• SPX to M4B
• SPX to MP4
• SPX to FLAC
• SPX to MP2
• SPX to MP3
• SPX to MPC
• SPX to OGG
• SPX to WAV
• SPX to WMA
• AAC to SPX
• AC3 to SPX
• AIFF to SPX
• APE to SPX
• FLAC to SPX
• M4A to SPX
• MP+ to SPX
• MP1 to SPX
• MP2 to SPX
• MP3 to SPX
• MP4 to SPX
• MPC to SPX
• MPP to SPX
• OFR to SPX
• OFS to SPX
• OGG to SPX
• TTA to SPX
• WAV to SPX
• WMA to SPX
• WV to SPX
Note that Audio Transcoder is not only a great Audio Converter. It is all-in-one solution to convert any audio files - WAV, WMA, OGG,MP3,MP4, AAC and many others, rip Audio CD's, edit metadata (audio tags) .
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