First used in cinemas, Dolby Digital is a digital audio signal that was placed on the film between the sprocket holes of the film, Today it is the standard audio format for DVDs. and provides the consumer with a audio system of up to 6 channels which is a perfect match for 5.1 channel amps.
Dolby Digital records, transfers and plays back each channel’s signals with all digital signals. In addition, each channel is recorded independently. Therefore, unlike the conventional analogue Pro Logic system, no signal is pushed into the two channels by matrix treatment. As a result, each channel’s divisional characteristics have been enhanced and stable playback has been achieved. Consumer-format Dolby Digital is basically the same as that for theatre use; only the data rate is different.

Dolby AC-3 was the name used for Dolby Digital prior to the late 1990s. It was designed as a versatile low bit ratecoder for’ Multchannel Digital Audio System’. Today AC-3 generally refers to sound system used on some laserdisc players produced prior to the advent of DVD. Although technically the same audio system, Laserdiscs used an AC-3 RF output that required the use of an amplifier with a AC-3 RF in or a converter to convert the RF signal to the more common SP-DIF format used today. The AC-3 RF output was purely digital RF audio output, the signal has to be decoded by a RF to digital decoder before it can be connected to the digital input of a Dolby Digital Amplifier. It was mostly older high end amplifiers that had a RF decoder input built in to the amplifier but now that laserdisc is a superseded format most high end amps sold today do away with the AC3 RF input in favor of more SP-DIF sockets used by DVD, set top boxes and bluray players.

The next version of Dolby Digital to come along was Dolby Digital EX. It extended the standard 5.1 channel Dolby Digital codec by matrixed rear channels, creating 6.1 or 7.1 channel output in a similar way to how prologic added surround channels to a stero sound track.

The latest version of Dolby Digital you may come across is Dolby TrueHD. Invented as a lossless audio codec it is was developed for the HD-DVD or Blu ray market and is a direct competitor to DTS-HD. It is a 7.1 channel system derived from 8 discretely recorded tracks. Dolby Digital Plus can also be found on Blu Ray or HD-DVD, it is similar in the fact that it is has 7.1 channels but it is considered a lossy audio format being derived from Dolby Digital. It would probably only be used on discs where there was not enough space to fit the TrueHD format.

The general rule of thumb is that if you buy an amp that supports the latest version of an audio format is that it will also support the earlier versions. If you have no intentions of buying a blu ray player and are quite happy with your 5.1 DVDs then there would be no need to upgrade from a Dolby Digital to a TrueHD amp as none of your media will be able to take advantage of the extra features of your new amp.