What happens when some audio is 5.1 and some is two channels? There are several ways to handle this scenario. One is to change the audio coding mode (acmod) metadata parameter inside the Dolby Digital (AC-3) encoder. This parameter describes the number of audio channels encoded in a Dolby Digital (AC-3) bitstream. The notation is very simple: front channels/rear channels/L(FE). For example, a 5.1 channel program is notated 3/2L representing Left, Right, and Center in the front, Left Surround and Right Surround in the rear, and L for an LFE, or Low Frequency Effects channel (the one that drives the subwoofer).
Changing the acmod parameter can be done in two ways: by changing the incoming metadata stream, or by recalling a preset in the encoder. The results are not equal, however. You will find that a metadata change of just this one parameter will produce a much smoother result in consumer equipment than recalling a preset. This is because in encoders such as the Dolby DP569, recalling a preset causes all of the metadata parameters to be updated and can cause an audible glitch in the program stream and may even upset downstream equipment.
Another way to handle two channel audio is to upmix or upconvert it to 5.1 channels and keep the acmod metadata parameter fixed at 3/2L or 5.1 channels. This method is gaining popularity because it covers any switching issues that might occur even by changing only the acmod parameter. It is even possible to have the upmixing process controlled by metadata and therefore be completely automated.
Either way, audio should be presented in as realistic a surround-field as possible with as few side effects as possible.
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