Almost all SD channels are DD2.0. (DD2.0 sound can be processed by Dolby Pro Logic II or other DSPs in your AVR to produce surround sound, but is not DD5.1. For most TV sources, DPLII is the appropriate setting for DD2.0 or "stereo" or "two channel" signals)
Most programming on HD channels is now DD5.1. There are usually enough channels with DD5.1 that switching to various channels should get you a DD5.1 signal from one of them, just as a test to see if your equipment is working properly.
HDPBS now has most of its programmes available in DD5.1. PBS Seattle must have an upmixer since they send out DD5.1 24/7. PBS Buffalo seems to be working properly now as the used to have only DD2.0. See link below.
CBC-HD has occasional programming in DD5.1.
As a general rule, most HD programming is now DD5.1 and most SD programming is DD2.0.
In order to receive DD5.1, you, of course, need a digital STB (Or TV) that has a digital audio output (coaxial or optical or HDMI). Your A/V receiver (or HTiB) also needs to have the equivalent digital input and a DD5.1 decoder. Such a receiver typically has "Dolby Digital" and/or DTS emblazoned on it. Many devices now have HDMI outputs and most Audio-Video Receivers (AVRs) now have HDMI inputs, however, the receiver must be able to handle audio via HDMI (some less expensive ones don't)
The STB or DVD player should usually be set to something like Dolby Digital or bitstream or raw or surround when using optical or coaxial audio connection. It can get tricky using HDMI connections, but most
times the HDMI/bitstream setting will work properly...
Some additional notes:
1. Some Channels have DD5.1 signal, but the audio is not properly encoded - the dialogue doesn't come out the center channel for example, coming out the Left Front & Right Front speakers (PBS and CBC). CBC, CBC affiliates and some PBS stations do this because they have older equipment that doesn't switch well between DD2.0 and DD5.1, so they send DD5.1 all the time, without themselves using a surround sound "upconverter", like some broadcasters do. Sometimes the dialogue is missing altogether. If dialogue is missing, use Closed Captioning. Some commercials on these channels may come in as properly encoded DD5.1.
2. Most streaming websites, apps, etc still have only DD2.0, although there are exceptions like Netflix.
3. DD5.1 channels are often lower in sound level than DD2.0 channels.
If you're not receiving DD5.1 on a particular station on a programme that "should" be DD5.1, then the issue may be with your affiliate. The service providers usually "pass" what they receive.
Often you may receive a DD5.1 signal, but the sound only comes out the LF & RF speakers. This indicates that the audio has been improperly encoded. If you want surround sound, you can listen to the analogue inputs and force DPL or you can set your STB to PCM instead of DD (if available). If dialogue is missing, you can also turn on CC.
4. Some AVRs/HTiBs do not handle the switch from DD2.0 (or PCM) to DD5.1 well, with clicks, pops or delays being common on some (especially less expensive) AVRs or HTiBs. Alter the settings or connections, or get a new AVR.
On many (older) HDTVs we've discussed, you cannot go "through" a TV and still get DD5.1 audio (many newer TVs may now have this functionality, while others may not pass a signal at all). You typically need to connect directly from the STB (or other device like a DVD/BD player) to the AVR and you need the correct settings in the STB (or other device - usually DD or bitstream). Sometimes there may be a firmware limitation when using HDMI for DD5.1.
The digital audio output on the back of many HDTVs is designed for use with the QAM/ATSC tuner (or SmartTV apps), not as a passthrough function, although many newer TVs may allow for passthrough functionality. This will give DD5.1 if properly configured for the internal ATSC/QAM tuner (or SmartTV Apps) for programming with DD5.1. You also typically need to turn off the TV's internal speakers to activate the audio output on the back of the TV. The proper setting is usually something like Dolby Digital, or bitstream or raw.
Some TVs may have a user setting under HDMI or Audio which needs to be changed to get DD5.1 instead of say PCM - the setting may be something like "passthrough".
Of course, you cannot get Dolby TrueHD or other lossless formats through the TV. For that you need an HDMI connection from the device (BD player) directly to the AVR
(or Multi-channel Analogue Audio, if available - rare these days)
See also the following more detailed discussion on the subject.
DD5.1 Discussion - And How It's "Handled" By Various Channels - Canadian TV, Computing and Home Theatre Forums
DD5.1 on various affiliates.
CNET Test of TVs with "passthrough". 20 TVs tested: Which sets can pass surround sound to a sound bar? - CNET
A lot of these are "older" but do give an indication that not all TVs will pass DD5.1.
Please PM me if you have any comments or suggestions.