This would depend on your receiver and settings, but normally mono audio should play through the center speaker only. If the left and right channels are different you will also hear them through the left and right front speakers.

Well it is a little bit what everybody said.

There is not necessarily a 1-1 correspondence between:

(1) What is actually on the DVD audio tracks
(2) What is said to be on the tracks (cover info, track info encoded on the DVD)
(3) What signal is actually sent to each speaker (see iPasta comments)
(4) What you can see by viewing the signals in the time domain

In fact:

(1) Only the recording engineer knows
(2) Can be wrong on the cover, or even on the DVD. Software such as DVDInfo (which DVDca is derived from), actually reads the encoding from the track info on the DVD. This information would likely have been supplied by the recording engineer.
(3) Depends on your home theater wiring and receiver settings.
(4) It would be virtually impossible to visually compare two audio waveforms in real time (I mean with your eyes - yes, software and/or hardware that can sample the signal can do it.)

An example of (3): You could incorrectly determine that a signal was "mono" if an error in your wiring sent both left and right channels the same signal (say the left channel), or if there is a "mono" setting on the receiver, artificially mixes a stereo signal down to mono by adding the two channels, and sending the sum to both speakers.

If you want a simple way to tell whether left and right signals coming from a stereo output are exactly the same, you can subtract them. If you have simple electronics skills, this is easy to do. Probably a ton of circuits on Google. If Left and Right are EXACTLY the same in amplitude at every instant in time, you will hear dead silence when the signals are subtracted ( a waveform will show a flat line). Theoretically it would be possible to do this in software while the DVD was playing, as Gunnar suggests.

Legacy receivers used to have a switch for this, and this method was sometimes used to synthesize pseudo surround effects; e.g,,  Left surround got (L-R), right surround got (R-L), and center got (L+R) in cheap equipment.

Ah, I almost forgot: the real issue is: what do the rules require that you submit? I think it is (2) - what is encoded on the DVD.

Quoting GSyren:

Quote:
2-channel mono will play through the left and right speakers, just like 2-channel stereo.

... what iPatsa meant: If you switch to Dolby, Dolby Surround or Dolby Prologic decoding, then a mono track is centered, while a Stereo track is mixed betwenn the three channels.

Yes, I found code in DVDPca/DVDInfo that handles the "AudioMode" descriptor on the DVD, which can have a value of "Dual Mono" for a channel count of 2. My understanding of Dual Mono, is that the same signal is designated for the both left and right channels.

                if (ChannelCount == 2)
                {
                    switch (AudioMode)
                    {
                        case TSAudioMode.DualMono:
                            description += " / Dual Mono";
                            break;

                        case TSAudioMode.Surround:
                            description += " / Dolby Surround";
                            break;
                    }
                }

Then there is "ChannelLayout" descriptor. Here is the code for that:
                  switch (ChannelLayout)
                    {
                        case TSChannelLayout.CHANNELLAYOUT_MONO:  // Send to center channel speaker.
                            description += "1.0";
                            break;
                        case TSChannelLayout.CHANNELLAYOUT_STEREO:
                            description += "2.0";
                            break;
                        case TSChannelLayout.CHANNELLAYOUT_MULTI:
                            description += "5.1";
                            break;
                    }

Now, you ask: is there such a thing as channel layout of "mono" for channel count = 2? Here is that code:

                if (ChannelLayout == TSChannelLayout.CHANNELLAYOUT_MONO &&
                    ChannelCount == 2)
                {
                }

Seems like even the original coders of DVDInfo punted on that one! 

I would encourage you to try DVDPca if you want to see what is encoded on the DVD. Back when I was testing, I think it was found to be very accurate.

By the way, just to be clear, the "encoding" I am talking about is the encoding of descriptive information (metadata) in the IFO files on the DVD, not the audio stream encoding.

My point that you forgot: how about check what is on the actual DVD (not the cover)? There is a placeholder in the specifications for somebody to tell you "Dual Mono". But there is no requirement for them to do so. If they used it, DVDPca and DVDInfo will display it. In those cases, no need to guess. Hmm, well I don't know if it is still working. Been a long time since the last update. Sorry about that. Guess I should do an overhaul and see what happens.

By the way, I just found out that VLC player actually can display waveforms. And I agree that it is impossible to say that ttwo waveforms are identical and therefore determine that it must be mono. But it is possible in many cases that waveforms are sufficiently different that you can tell with a fair degree of certainty that it is stereo.

Not sure how useful it is, though, since you would have to look for places where the tracks really are quite different.

Quoting GSyren:

Quote:
The problem isn't differentiating between 1-channel and 2-channel.
The problem is differentiating between 2-channel mono and 2-channel stereo.
2-channel mono will play through the left and right speakers, just like 2-channel stereo.

Switch to Dolby Prologic Decoding. It will most certainly play 2-channel mono through the center speaker and 2-channel stereo through the left and right speakers.

Quoting GSyren:

Quote:
"How did Ken envision that we would tell 1-channel and 2-channel mono apart?"

I daresay he didn't envision that at all. I think he really didn't expect anything beyond entering what the cover says. It's just unfortunate that cover information is so often completely inaccurate.