Greenwich Time Signal broadcasts the pips every hour. Pips can also be heard on many BBC Local Radio stations although their use is up to the discretion of individual stations. A rare quarter-hour Greenwich Time Signal can be heard at 05:15 weekdays on Wally Webb's programme on six BBC Local Radio stations in the east of England, as part of his "sychronised cup of tea" feature. In 1999, pip-like sounds were incorporated into the themes written by composer David Lowe to introduce BBC Television News programmes. They are still used today on BBC One, BBC World News and BBC News. The BBC does
Time reversal signal processing time reversal (using a negative time) of that solution is also a solution. This occurs because the standard wave equation only contains even order derivatives. Some media are not reciprocal (e.g. very lossy or noisy media), but many very useful ones are approximately so, including sound waves in water or air, ultrasonic waves in human bodies, and electromagnetic waves in free space. The medium must also be approximately linear. Time reversal techniques can be modeled as a matched filter. If a delta function is the original signal, then the received signal at the TRM is the impulse response of the