Talk:Videos/A Digital Media Primer For Geeks

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(Video vegetables (they're good for you!): chroma samples positioning)
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::Thanks Dryo ! that's what I thought. Then I don't quite understand what this chroma samples positioning/siting is about. Is it actually defining the algorithm you should use to compute RGB pixels from YUV samples ? Is is defining the influence zone of chroma samples over luminance ones ? What I don't get is how you can talk about spatial positioning for something that is, well... not spatial (samples). Thank you again ! --[[User:Ledahulevogyre|Ledahulevogyre]] 09:52, 25 September 2010 (UTC)
::Thanks Dryo ! that's what I thought. Then I don't quite understand what this chroma samples positioning/siting is about. Is it actually defining the algorithm you should use to compute RGB pixels from YUV samples ? Is is defining the influence zone of chroma samples over luminance ones ? What I don't get is how you can talk about spatial positioning for something that is, well... not spatial (samples). Thank you again ! --[[User:Ledahulevogyre|Ledahulevogyre]] 09:52, 25 September 2010 (UTC)
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:::Imagine a small 2x2 image, with the top two pixels blue, and the bottom two pixels red. Luminance will be sampled at each pixel, but (for 4:2:0), only one sample of Cr will be taken for this 2x2 set, so you'll have to decide where. If you place the sample on the middle horizontally, but aligned with every even or odd line, you'll get a sample from either blue, or red. If you place the sample horizontally and vertically, you'll get a sample from pink. Similarly for each other possible placement algorithm. [[User:Ogg.k.ogg.k|Ogg.k.ogg.k]] 10:24, 25 September 2010 (UTC)
==Containers==
==Containers==

Revision as of 10:24, 25 September 2010

Welcome to the discussion.

To discuss the video, make an account and hit edit. Please feel free to point out errata, suggested additional resources, or just ask questions!


Contents

Introduction

Analog vs Digital

Raw (digital audio) meat

Don't forget when talking about higher sampling rates that frequency and temporal response are inherently linked. One often overlooked aspect of this is the value of higher sampling rates in presenting subtle differences in multi-channel timing (e.g. the stereo field). Even fairly uncritical listeners presented sample audio blind can notice this. --Chaboud

They aren't merely "technically linked". They're mathematically indistinguishable. If a system doesn't has a response beyond some frequency it also lacks time resolution beyond some point.
To the best of my knowledge a perceptually justified need for higher rates is not supported by the available science on the subject. Not only is there no real physiological mechanism proposed for this kind of sensitivity, well controlled blind listening tests don't support it— well controlled being key, loudspeakers can suffer from considerable non-linear effects including intermodulation, and having a lot of otherwise inaudible ultrasonics can produce audible distortion at lower frequencies. Another common error is running the DAC at different frequencies— with the obvious interactions with the reconstruction and analog filters. A correct test for determining the audibility differences of higher sample rates needs to use a single DAC stage at the highest frequency, re-sampling digitally to create the bandpass... etc. I'm not aware of any such test supporting a need for information beyond 24kHz.
I normally suggest to people looking for increased to look into acoustic holography techniques like higher-order ambisonics and wavefield synthesis.
The beyond 48kHz sampling subject subject has been discussed a number of times on hydrogen audio, I recommend reading the thread there. They are quite informative. Most audio groups out there online and off are not very scientifically oriented (e.g. evidence based)— HA is special because it is one of the few that are.--Gmaxwell 06:00, 24 September 2010 (UTC)

Video vegetables (they're good for you!)

An interesting point is that the discussion of the linear segment in the normal display responses (e.g. sRGB) is incorrect, or at best incomplete, though I've coming up short on good citations for this, so Wikipedia remains uncorrected at this time.--Gmaxwell 05:15, 22 September 2010 (UTC)


Hi there, great tutorial, but in fact the most common DVD standard is 720 pixels by 480 pixels, with a pixel ratio of 0.9, yielding a device aspect ratio of 1.35. I understand that you're trying to simplify the lecture to 4:3 aspect (1.333) for newbies, I think this is ultimately misleading, since the vast majority of DVDs are not sampled at 704x480. --Dryo

Sort of-- the most common encoding is 720x480, but with the crop area set to 704x480; that's what the standard calls for (I was being sneaky when I said 'display resolution of 704x480'). Many software players ignore the crop rectangle and also display the horizontal overscan area. Many software encoders also just blindly encode 720x480 without setting the crop area. It is a source of *much* confusion. --Monty
"The standard" here being— Rec. 601? Is there anything else? We should probably at least link Wikipedia:overscan. --Gmaxwell 13:13, 24 September 2010 (UTC)
OK, thanks for the clarification Monty... I did not even know that the horizontal crop area existed.

"[...] most displays use [RGB] colors [...]". Doesn't that sentence contradict this one : "[...] video usually is represented as a [...] luma channel along with additional [...] chroma channels, the color". I don't understand what "position the chroma pixels" means exactly. Are we talking of real points on a display ? Thanks, great video ! --Ledahulevogyre 13:59, 24 September 2010 (UTC)

Display devices use RGB. Most video is actually encoded as YUV, luma plus two color "difference" channels. This reduces the bandwidth of raw video by cleverly exploiting limitations in human perception. Additionally, color samples need not be as frequent as luminance samples. So "chroma pixels" are the color data samples, not the pixels on a real display. --Dryo
Thanks Dryo ! that's what I thought. Then I don't quite understand what this chroma samples positioning/siting is about. Is it actually defining the algorithm you should use to compute RGB pixels from YUV samples ? Is is defining the influence zone of chroma samples over luminance ones ? What I don't get is how you can talk about spatial positioning for something that is, well... not spatial (samples). Thank you again ! --Ledahulevogyre 09:52, 25 September 2010 (UTC)
Imagine a small 2x2 image, with the top two pixels blue, and the bottom two pixels red. Luminance will be sampled at each pixel, but (for 4:2:0), only one sample of Cr will be taken for this 2x2 set, so you'll have to decide where. If you place the sample on the middle horizontally, but aligned with every even or odd line, you'll get a sample from either blue, or red. If you place the sample horizontally and vertically, you'll get a sample from pink. Similarly for each other possible placement algorithm. Ogg.k.ogg.k 10:24, 25 September 2010 (UTC)

Containers

General discussion

The video hasn't yet been formally released but we have all the sites up early in order to get everything debugged... Feedback on site functionality prior to the official release would be very helpful. --Gmaxwell 15:15, 22 September 2010 (UTC)

Released now, but still tell us about bugs :-) --Monty


Atom/RSS feed

Could not find an Atom/RSS feed for the video episodes. A videocast url with video-link enclosures would be ideal for getting future episodes. But even a announce-only feed would be convenient to track new episode releases. --Gsauthof 17:41, 24 September 2010 (UTC)

One does not exist yet— as a stopgap you can follow the Xiph tag on Monty's blog and you'll be sure to hear about new videos. This has to be the most requested feature— I'll make sure we do it before the next video.--Gmaxwell 20:50, 24 September 2010 (UTC)

44100 Hz Trivia

The reason CDs use a 44,100 Hz (actually 44,056 Hz in the United States) is because, before dedicated digital recorders became mainstream, the only way a recording engineer or producer could record digital audio was with a piece of gear called a "PCM processor" or a "PCM Adaptor" (like a Sony PCM-F1 of PCM-501). These would take an audio input and, after running through the A/D if necessary, it would modulate it onto a baseband monochrome NTSC or PAL video signal that could then be recorded onto a 3/4" U-Matic video tape. The processors would accept two inputs, at 16 bits, giving a total bit rate of 1411200 bps. This number has the serendipitous property of being evenly divisible by both 30 and 25, 47040 and 56448, and these numbers allow both NTSC and PAL to encode the same number of bits, 98, per scan line (with the NTSC 480 line raster and PAL 576 line raster). It was just convenient selection of integers. CDs would be recorded at 44.1k in Europe as they were mastered onto 25 fps tapes, while CDs in the US were recorded at a "nominal" 30fps were actually at 44.056, but the difference in tone is basically inaudible. Iluvcapra 18:44, 24 September 2010 (UTC)

Note that the PCM audio signal, once modulated to NTSC or PAL, can be recorded on any video recorder, not just U-matic. The most common tape format for PCM audio was Sony Betamax. Sony sold Betamax decks bundled with external PCM A/D converter units for the pro audio market. The PCM-F1 was designed to be used with Betacam VCRs. -- Dryo
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