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The Q4401 Variable Rate Vocoder is a full‑duplex speech encoder and decoder that provides near toll‑quality speech at data rates as low as 800 bps. It supports both fixed‑rate (4, 4.8, 8, 9.6 kbps) and variable‑rate modes, automatically adjusting the rate every 20 ms frame based on speech energy.
The device is controlled by an external processor through an 8‑bit parallel interface and exchanges 25‑byte packets every 20 ms. It includes optional features such as input/output muting, volume control, tone generation, VOX (hands‑free) mode, and loop‑back functions. Power consumption in Sleep Mode is under 50 mW.
| Fixed‑rate data rates | 4, 4.8, 8, 9.6 kbps |
|---|---|
| Variable‑rate data‑rate range | 800 bps – 8 kbps (Normal) or 800 bps – 9.6 kbps (Enhanced) |
| Packet size per frame | 25 bytes |
| Frame timing | 20 ms frames, 160 PCM samples per frame |
It can encode speech at 4, 4.8, 8, or 9.6 kbps in Fixed‑Rate Mode.
Sleep Mode consumes less than 50 mW, about 10 % of normal operation power.
VOX mode enables hands‑free operation by allowing only one side of a full‑duplex link to transmit when voice activity is detected, reducing echo.