With my receiver (a Kenwood) there is a Auto Detect mode and just by switching to a channel with DD, it changes by itself. I have seen movies that say they are in DD and are actually not. Check to see if you have an auto digital mode or something like that.
I have the Denon 3801 and had the 3600 before that. They both display the mode in the window. My 3801 says "Pro Logic" on most DTV/OTA content, but when the source is DD5.1, it says "Dolby Dig". It will also say "DTS ES 6.1" and so on when playing DVD's. The best way to set it up is to enable the "Auto Detect" mode on the input. That way it can sense what is coming in and switch to that mode automatically. I assume the 5800 has this feature.
FYI, my receiver also has the "Pro Logic" and "Dolby Digital" titles that pop up, but note that sometimes the receiver will read a DD 5.1 signal, but the station will be broadcasting no sound over some of the channels. The station may have an impulse going through all channels, but sometimes sound is limited to only a few channels.
Dolby Digital, while commonly referred to as "5.1", actually incorporates eight different coding modes ranging from "1/0" (mono) to "2/0" (stereo) up to "3/2L" (aka 5.1). It is up to the network to configure their audio signal for the appropriate coding mode to reflect the number of channels in the broadcast program. In current practice, while Dolby Digital can carry anywhere from a single mono signal up to a full 5.1 theatrical presentation, there's no guarantee that all 5.1 channels are carrying audio, and there is one chief reason for this.
In this infancy of digital television broadcast, most networks are using a "pass through" model, where the network in New York (or wherever) is sending the local affiliate an ATSC compliant "transmission" stream that is simply passed through to the consumer at home (hence the term "tranmission" since it's prepackaged for delivery to the home). In these instances, the default channel mode may be 5.1 since the featured presentation may be a 5.1 broadcast while commercials and station breaks will most likely be stereo at best. The coding mode will not change between the program and the station break, and you'll still see "5.1" displayed on your receiver/decoder even though the only active channels will be the left and right main channels at times.
FYI, there is no bandwidth savings when a 5.1 DD stream is encoded with only two active channels. There is really no reason for the networks to broadcast a stereo program in a 5.1 mode, except that it is easier to pass through a 5.1 network feed rather than locally changing the channel mode to reflect the true number of active audio channels within a program.
Until such time as the local folks purchase the necessary equipment to create their own multichannel audio programs, and the networks transmit an ATSC compliant "distribution" stream to the affiliates (this is more like the current analog broadcast model in use for many years where the network feed is received overnight and saved for rebroadcast after inserting local ads and news bumps), the most common model for DTV broadcast will be "pass through", and consumers will continue to experience some level of confusion regarding the audio content of a program.
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