Variable data rate receiver
View Patent ↗A clock signal and data is recovered from a variable rate signal including biphase mark encoded digital audio data. Such recovery involves regularly determining a minimum or maximum pulse width in the biphase mark encoded digital audio data using a high frequency clock. This pulse width is used to define a window in which a transition in the biphase mark encoded digital audio data may be detected. If a transition occurs in the defined window, a data one is output; if a transition does not occur in the defined window, a data zero is output. The recovered clock has a period of twice the minimum pulse width. A minimum or maximum pulse width can be tracked with an accumulator with decay. In particular, if the data rate of the input signal becomes faster, the shortest pulse will become shorter and a minimum value stored by the accumulator will become shorter. If the data rate of the input signal becomes slower, the longest pulse will become longer and any maximum value stored by the accumulator will become longer. However, if a minimum is stored and the input signal becomes slower, or if a maximum is stored and the input signal becomes faster, the stored minimum or maximum values would not otherwise change. By incorporating decay, the stored value changes over time towards a larger value (if a minimum is stored) or a smaller value (if a maximum is stored) to a limit. By regularly updating the determined minimum or maximum pulse width, the rate of the input signal may vary over time and still permit recovery of the clock and data signals. By using a window in which a transition is detected, jitter in the input signal is tolerated and gradual changes in input rate may be tracked.
1. Apparatus for recovering a clock and data from a variable rate signal including biphase mark encoded digital audio data, comprising:
an edge detector having an input for receiving the variable rate signal and an output providing an edge signal indicative of points in time in the variable rate signal at which a transition in signal level occurs;
a counter having a reset input connected to receive the edge signal from the edge detector, an increment input connected to receive a high speed clock signal and an output providing a count that increments at the rate of the high speed clock signal;
logic for identifying an extremum pulse width from the counter over a period of time; and
a clock and data generator responsive to the extremum pulse width and the edge signal to generate a recovered clock and to provide recovered data in synchronization with the recovered clock.
2. The apparatus of claim 1 , wherein the clock and data generator includes
an adaptive windowing arithmetic logic unit for specifying a window using the extremum pulse width; and
a bitclock and data recovery block for determining whether a transition occurs during the specified window and generating the recovered data based on the determination.
3. The apparatus of claim 2 , wherein the logic for identifying an extremum pulse width includes an accumulator for updating the extremum pulse width for each edge in the input signal, and for decaying the extremum pulse width over time.
4. The apparatus of claim 1 , wherein the logic for identifying an extremum pulse width includes an accumulator for updating the extremum pulse width for each edge in the input signal, and for decaying the extremum pulse width over time.
5. The apparatus of claim 1 , wherein the extremum is a minimum.
6. A method for recovering a clock and data from a variable rate signal including biphase mark encoded digital audio data, comprising:
regularly identifying an extremum pulse width in the variable rate signal;
specifying a window using the extremum pulse width;
determining whether a transition occurs during the specified window; and
generating a recovered clock and recovered data in synchronization with the recovered clock based on the determination.
7. The method of claim 6 , wherein regularly identifying an extremum pulse width comprises:
detecting transitions in the variable rate signal;
counting pulses of an over-sampling clock between detected transitions; and
storing an extremum of the counted pulses for each transition over a period of time.
8. The method of claim 7 , wherein storing comprises:
updating the extremum pulse width for each transition in the variable rate signal; and
decaying the extremum pulse width over time.