This invention relates to delay lines and, in particular to a delay line including a coarse delay and a fine delay to provide high resolution over relatively long periods with relatively few stages of delay.
In telephone systems and elsewhere, it is often desired to measure or match a particular delay, e.g. for echo cancellation. The distance that a signal is traveling causes a minimum delay. Digital calling apparatus further delays a signal in the digitizing process and in the batch (packet) mode that signals are often handled. Using a satellite relay can add considerably to the delay; a minimum of 250 milliseconds each way.
An echo is perceived if a delay is greater than 20-50 milliseconds. Digital packet transmission through a satellite can produce a delay in excess of 600 milliseconds. Modern network equipment is incapable of handling a delay longer than about 100 milliseconds. Acoustic delays, such as reverberations in a room, can be even longer, up to 1,500 milliseconds.
The sampling rate of analog to digital (A/D) converters in telephone systems is typically 8,000 samples per second. This number was chosen because of the relatively narrow bandwidth of a telephone system, 300-3,400 Hz, and because of the speed limitations of digital signal processing (DSP) devices. At 8,000 samples per second, the samples are separated by 125 microseconds and a 3.4 kilohertz signal is sampled only 2.3 times per cycle. This is not particularly good resolution.
In order to increase resolution, one must increase the number of samples, which causes a corresponding increase in the number of storage sites. The number of storage sites is limited by the cost of manufacturing suitable integrated circuits and the complexity of addressing the sites in real time.
In an analog system, the signals are not converted to digital data, which simplifies the circuitry. However, the storage time for the samples is presently limited by the characteristics of the storage node to approximately one half second without some sort of refreshing. For longer storage times, A/D conversion and memory storage are necessary.
A large number of storage sites adversely affects the time for the system to lock onto the delay, referred to herein as convergence. In a constantly changing environment, such as a telephone, system delays can change during a call and acoustic delays can change during a call because a person moves about a room. In the prior art, the settings for an echo canceling circuit are not changed during a call, largely due to a long convergence time.
In view of the foregoing, it is therefore an object of the invention to provide a high resolution delay line using relatively few storage elements or delay elements.
Another object of the invention is to provide a high resolution delay line that enables a system incorporating the delay line to converge quickly.
A further object of the invention is to provide a high resolution delay line capable of delaying a signal for one second or more.
Another object of the invention is to provide a high resolution delay line that can be implemented in either analog form, digital form, or a mixture of the two.