Ideas on how to test if video quality affects perceived audio quality?, [was “Scientific method to use (College Dissertation Help!)”] Options

[MikeyTSH]

Not sure if this is the right forum to post this in, if not, feel free to move the topic and apologies mods!

Anyhoo, I'm in University in England doing my final year dissertation. I want to see if visual stimuli affects the perception of audio quality. The plan is to subject test subjects to WAV, MP3 and AAC files with the latter two encoded at 320, 256, 128 and 96. After ABX testing the MP3 to the AAC at each bit rate, I then want to throw video in at 1080p, 720p, 480p and 240p. Test subjects will do an ABX on the audio but I want to see if a corresponding/conflicting video quality will affect their choices.

An idea I had was the have the audio quality drop throughout the video too but I'm having real trouble working out the best way to test this. I've already made a test "scaling" file, simply putting cross-fades between each file to make the transition as seamless as possible but my big question is really what testing method would I use here? I can hardly say "which do you think sounds better" because the audio is going to degrade in quality. I really want to ask them to hit the pause button when they believe the audio quality has changed but I'm worried about 2 things - 1) will this cause preemptive pressing of the button 2) if I can get around this, what kind of testing method would I use? The idea for this test is to have 2 video files - one remaining at 1080p and then another that degrades in time with the speed that the audio quality degrades, to link the visual stimuli hypothesis. But here again I hit a brick wall - do I ask either a) when did the audio quality change or b) do you think the audio quality changed, in addition to the question "which one sounded better" - my lecturer suggested that having both MP3 and AAC can cement my hypothesis in BSc territory as I can directly hypothesize based on how the codec "technically" work.

Any and all input is greatly appreciated, I look on the internet for some answers, end up going, for example, "oh, null testing, what's that?" and then 20 links later I'm down a rabbit hole that has me scratching my head and, unfortunately, procrastinating.

Thanks again!
Mike

This post has been edited by MikeyTSH: Jan 14 2013, 01:07

[MikeyTSH]

I think I'll drop the variable one but as far as encoding goes I'm using LAME for MP3 and iTunes' encoder for AAC - I want to keep it as close to consumer style encoding as possible - I'll have to check and see what kind of encoding Amazon MP3 use and get as close as I can to that.

Thanks so much for the response, it's all incredibly helpful - as I've said, I find myself going down a rabbit hole that seems endless!

[db1989]

I'll have to check and see what kind of encoding Amazon MP3 use and get as close as I can to that.

Amazon seems to be quite inconsistent in using various versions and configurations of LAME, at least last I heard. There have been a discussion or two here about this in the past; I don’t have links to hand, but you should be able to find them via a search and may find them useful.