Speech decoding in a zero BER environment

In a receiver, a method for decoding a rate-1/2 convolutionally encoded signal and a rate-1/4 convolutionally encoded signal to produce a decoded signal, the rate-1/2 convolutionally encoded signal encoded by a first transfer function and the rate-1/4 convolutionally encoded signal encoded by a second transfer function the method comprising the steps of:

• processing the rate-1/2 convolutionally encoded signal with a third transfer function to generate a first output signal;
• processing the rate-1/2 convolutionally encoded signal with a fourth transfer function to generate a second output signal;
• processing the rate-1/4 convolutionally encoded signal with a fifth transfer function to generate a third output signal;
• processing the rate-1/4 convolutionally encoded signal with a sixth transfer function to generate a fourth output signal;
• saving the first and fourth output signals;
• combining the first and second output signals to produce a first error signal;
• combining the third and fourth output signals to produce a second error signal;
• using the first saved output signal as the decoded signal when the first error signal indicates a number of errors equal to zero;
• and
• using the fourth saved output signal as the decoded signal when the first error signal indicates a number of errors greater than zero and the second error signal indicates a number of errors equal to zero.

A communication system comprising:

• a transmitter for generating a rate-1/2 convolutionally encoded signal and a rate-1/4 convolutionally encoded signal, the rate-1/2 convolutionally encoded signal encoded by a first transfer function and the rate-1/4 convolutionally encoded signal encoded by a second transfer function;
• and
• a receiver for decoding the rate-1/2 convolutionally encoded signal and the rate-1/4 convolutionally encoded signal to produce a decoded signal, the receiver including:
• a first inverse convolutional encoder for processing the rate-1/2 convolutionally encoded signal with a third transfer function, thus generating a first output signal;
• a second inverse convolutional encoder for processing the rate-1/2 convolutionally encoded signal with a fourth transfer function, thus generating a second output signal;
• a third inverse convolutional encoder for processing the rate-1/4 convolutionally encoded signal with a fifth transfer function, thus generating a third output signal;
• a fourth inverse convolutional encoder for processing the rate-1/4 convolutionally encoded signal with a sixth transfer function, thus generating a fourth output signal;
• a first memory for storing the first output signal;
• a second memory for storing the fourth output signal;
• a first signal combiner for generating a first error signal in response to the first and second output signals, wherein the first output signal is used as the decoded signal when the first error signal indicates a number of errors is equal to zero;
• and
• a second signal combiner for generating a second error signal in response to the third and fourth output signals, wherein the fourth output signal is used as the decoded signal when the first error signal indicates the number of errors is greater than zero, and the second error signal indicates that the number of errors is equal to zero.