Jitter-free sample rate conversion
View Patent ↗A method and apparatus are described for asynchronous sample rate conversion, in particular those which use an interpolating filter, especially a polyphase interpolating filter (FB 4 ). The input and output signals have jitter but the polyphase branch signals have no or reduced jitter due to the operation of a jitter removing means such as a phase locked loop. The jitter is reduced only in the polyphase branch signals. Various methods are described for reducing the jitter.
1. An apparatus for converting an input time discrete signal with an input sampling rate to an output signal having an output sampling rate, the conversion ratio being N, the conversion ratio being the ratio of the output to the input sampling rate, the apparatus receiving a jittery input timing signal comprising:
polyphase filter means receiving the input signal and for outputting samples for the output signal,
control means receiving the jittery input timing signal and an output timing signal and for providing the filter means with polyphase branch selector data, and
means for reducing jitter in the polyphase branch selector data.
2. The apparatus according to claim 1 , wherein the means for reducing jitter in the polyphase branch selector data includes a phase-locked loop.
3. The apparatus according to claim 1 , wherein the means for reducing jitter (PLL 6 ) in the polyphase branch selector data includes a means for leveling a difference signal between the input timing signal and the output timing signal.
4. The apparatus according to claim 1 , the apparatus having means for determining the conversion ratio from the polyphase branch data, and wherein the means for reducing jitter reduces jitter only in the polyphase branch selector data.
5. The apparatus according to claim 1 , wherein the apparatus is implemented in software running on a processing engine.
6. An apparatus to convert an input time discrete signal having an input sampling rate to an output signal having an output sampling rate, at a conversion ratio N of the output to the input sampling rate, the apparatus comprising:
a polyphase filter to receive the input signal and to output samples for the output signal;
a controller to receive a jittery input timing signal and an output timing signal, and to provide the filter with polyphase branch selector data; and
a reducer to reduce jitter in the polyphase branch selector data; and
a converter arrangement to determine the conversion ratio N according to
N=S [n] −S [n−1] , and
S
[
n
]
=
C
B
[
n
]
-
C
A
[
m
]
C
A
[
m
+
1
]
-
C
A
[
m
]
,
where
C B[n] ≦C A[m] ≦C B[n+1] and T A ≧T B and where
C X[n] is the time of the nth clock pulse on a clock C X and T X is the period of the clock C X .
7. An apparatus for sample rate converting a number of M input streams, sampled with M sampling clocks to M output streams sampled with M other sampling clocks, the conversion ratio being N, the conversion ratio being the ratio of the output to the input sampling rate, the apparatus receiving at least one jittery input timing signal, comprising: polyphase filter means receiving an input time discrete signal and for outputting samples for the output signal, control means receiving the jittery input timing signal and an output timing signal and for providing the filter means with polyphase branch selector data, and means (PLL 6 ) for reducing jitter in the polyphase branch selector data.
8. The apparatus according to claim 7 , running on a digital signal processor.
9. The apparatus according to claim 7 , wherein the sample rate conversion of each input stream is independent of the conversion of all the other conversions.
10. The apparatus according to claim 7 , wherein the means for reducing jitter in the polyphase branch selector data includes a phase-locked loop.
11. The apparatus according to claim 7 , wherein the means for reducing jitter in the polyphase branch selector data includes a means for leveling a difference signal between the input timing signal and the output timing signal.
12. The apparatus according to claim 7 , the apparatus having means for determining the conversion ratio from the polyphase branch data.
13. The apparatus according to claim 7 , wherein the means for determining the conversion ratio N determines according to:
N=S [n] −S [n−1]
and
S
[
n
]
=
C
B
[
n
]
-
C
A
[
m
]
C
A
[
m
+
1
]
-
C
A
[
m
]
where C B[n] ≦C A[m] ≦C B[n+1] and T A ≧T B and where
C X[n] is the time of the nth clock pulse on a clock C X and T X is the period of the clock C X .
14. A method of converting an input time discrete signal with an input sampling rate to an output signal having an output sampling rate, the conversion ratio being N, the conversion ratio being the ratio of the output to the input sampling rate, the method comprising:
receiving a jittery input timing signal and an output timing signal,
receiving the input time discrete signal,
polyphase filtering the input data signal to output samples for the output signal,
providing polyphase branch selector data for the polyphase filtering step, and
reducing jitter in the polyphase branch selector data.
15. The method according to claim 14 , wherein reducing jitter in the polyphase branch selector data includes regularizing the polyphase branch selector data using a phase-locked loop.
16. The method according to claim 14 , wherein reducing jitter in the polyphase branch selector data includes leveling a difference signal between the input timing signal and the output timing signal.
17. The method according to claim 14 , further comprising determining the conversion ratio from the polyphase branch data, and wherein the step of reducing jitter includes reducing jitter only in the polyphase branch selector data.
18. The method according to claim 14 , further comprising determining the conversion ratio N according to
N=S [n] −S [n−1]
and
S
[
n
]
=
C
B
[
n
]
-
C
A
[
m
]
C
A
[
m
+
1
]
-
C
A
[
m
]
where C B[n] ≦C A[m] ≦C B[n+1] and T A ≧T B and where
C X[n] is the time of the nth clock pulse on a clock C X and T X is the period of the clock C X .
19. A method of sample rate converting a number of M input streams, sampled with M sampling clocks to M output streams sampled with M other sampling clocks, the conversion ratio being N, the conversion ratio being the ratio of the output to the input sampling rate, the method comprising:
receiving at least one jittery input timing signal and at least one output timing signal,
receiving an input time discrete signal and polyphase filtering the input signal and outputting samples for the output signal,
providing the polyphase filtering step with polyphase branch selector data,
receiving the jittery input timing signal and the output timing signal and reducing jitter in the polyphase branch selector data.
20. The method according to claim 19 , wherein the sample rate conversion of each input stream is independent of the conversion of all the other conversions.