# US 5,708,686 A · Method for receiver-side clock recovery for digital signals

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- Number: US 5,708,686 A
- Document ID: US5708686A
- Status: Granted
- Kind: A
- Application number: 08/618,437
- Filed: 1996-03-15
- Granted: 1998-01-13
- Earliest claimed: 1995-03-16
- Assignees: Deutsche Telekom AG
- Inventors: Ulf Assmus; Willy Heckwolf; Detlef Martin; Dieter Becker
- Web page: https://patentgrep.com/patent/US5708686A

## CPC

- H04J 3/0632: {Synchronisation of packets and cells, e.g. transmission of voice via a packet network, circuit emulation service [CES] (queuing arrangements in packet switching elements H04L49/90; synchronising systems for the synchronous transmission of a pulse code modulated video signal with one or more other pulse code modulated signals H04N7/56)}
- H04L 2012/5616: {Terminal equipment, e.g. codecs, synch.}
- H04L 2012/5674: {Synchronisation, timing recovery or alignment}
- H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S)
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

In a method for receiver-side clock recovery for digital signals having a constant bit rate following cell-structured, asynchronous transmission with pauses of different length between individual cells using the loading state of an FIFO memory into which the received digital signals are written, at the start of a transmission the digital signals are initially read with a received clock into the FIFO memory holding multiple cells of the received signals until the FIFO memory is half filled. The digital signals written into the FIFO memory are read out with a readout clock whose frequency is smaller than the frequency of the received clock. During the readout a signal for controlling the frequency of the readout clock is derived from the respective loading state of the FIFO memory.

## Claims

### Claim 1

A method for receiver-side clock recovery for digital signals having a constant bit rate following cell-structured, asynchronous transmission, the method comprising the steps of:
initially reading digital signals into a FIFO memory with a received clock until the FIFO memory is half-filled;
generating a half-full signal whenever the FIFO memory is a half-full state;
reading out the digital signals stored in the FIFO memory with a readout clock whose frequency is lower than the frequency of the received clock;
generating during readout a signal for controlling the frequency of the readout clock, the readout clock frequency derived from a loading state of the FIFO memory; and
setting a counter corresponding to the loading state of the FIFO memory to a half counter state in response to the half-full signal.

### Claim 2 (depends on 1)

The method as recited in claim 1 further comprising the step of generating a control variable based on the loading state of the FIFO memory.

### Claim 3 (depends on 2)

The method as recited in claim 2 wherein the control variable controls at least one voltage-controlled oscillator from whose output signal the readout clock frequency is derived.

### Claim 4 (depends on 3)

The method as recited in claim 3 wherein the control variable is fed to multiple voltage-controlled oscillators.

### Claim 5 (depends on 1)

The method as recited in claim 1 further comprising the step of querying the loading state of the FIFO memory at time intervals sufficiently large to provide a smoothing of the readout clock.

### Claim 6 (depends on 5)

The method as recited in claim 5 further comprising the step of determining a point in time for querying the loading state of the FIFO memory through a cell clock obtained from the received digital signals.

### Claim 7 (depends on 5)

The method as recited in claim 5 further comprising the step of forming an average value of multiple queries of the loading state of the FIFO memory.

### Claim 8

A method for receiver-side clock recovery for digital signals having a constant bit rate following cell-structured, asynchronous transmission, the method comprising the steps of:
initially reading digital signals into a FIFO memory with a received clock until the memory is filled to a predetermined level;
reading out the digital signals stored in the FIFO memory with a readout clock whose frequency is lower than the frequency of the received clock;
generating daring readout a signal for controlling the frequency of the readout clock, the readout clock frequency derived from a loading state of the FIFO memory; and
forming an average value of the loading state of the FIFO memory through a number of queries of the loading state of the FIFO memory.

### Claim 9

A digital signal transmission system comprising:
a memory for reading a received clock and digital signals having a constant bit rate following cell-structured, asynchronous transmission;
a signal averager receiving an input from the memory, the signal averager for averaging multiple values of a loading state of the memory;
a clock generator receiving an input from the signal averager, the clock generator receiving an input from the signal averager; and
a central clock providing timing signals to the memory, signal averager, and clock generator
the memory being a FIFO memory.

### Claim 10 (depends on 9)

The digital signal transmission system as recited in claim 9 wherein the clock generator generates an output clock signal and comprises a buffer memory, the buffer memory receiving data from the memory and the output clock signal.

### Claim 11 (depends on 9)

The digital signal transmission system as recited in claim 9 wherein the signal averager has an adder and a divider.

### Claim 12 (depends on 9)

The digital signal transmission system as recited in claim 9 wherein the clock generator comprises a controllable frequency divider, the frequency divider being connected after a voltage-controlled oscillator to permit a selectable frequency of an output of the clock generator.

### Claim 13 (depends on 12)

The digital signal transmission system as recited in claim 12 wherein the frequency divider is controllable depending on control data received with the digital signals.

### Claim 14 (depends on 12)

The digital signal transmission system as recited in claim 12 wherein an internal clock is a function of the signal of the voltage-controlled oscillator.

### Claim 15 (depends on 12)

The digital signal transmission system as recited in claim 12 wherein the frequency divider is controllable depending on a loading state of the memory.

### Claim 16 (depends on 9)

The digital signal transmission system as recited in claim 9 further comprising a cell discriminator, the memory being arranged at an input of the cell discriminator.