video DAC question

[Matt Broeska]

hey guys,

i was looking at the box for my toshiba 2109 just last night and noticed that it says it has a 10 bit, 27 mhz video dac. now, correct me if i'm wrong, but i thought i read recently that the new denon 2800 has a 12 bit, 54 mhz video dac. so i have 2 questions i guess... one: does the increased mhz make for a major difference in image quality and is it in reference to processing speed (sort of like a 500 mhz vs. an 800 mhz processor in a computer)? and two: is the difference in 10 and 12 bit in reference to colour depth per chanel like the difference between an 8 and 16 bit image? would 2 bits per chanel actually make for a huge difference in image quality? i know that it's a progressive vs. non-progessive player, but if it's a colour issue, how noticeable is it?

thanks in advance for any answers...

[Lexman]

Matt, interesting questions. I wish I had the answers, but I don't. Perhaps we will find the answer, or at least part of the answer yet...

Lex





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[Matt Broeska]

another thing i could add to this is, at what bit depth are dvds encoded? i would assume 8 bits per chanel... 16 bit would be a significant increase in the size of the mpeg and i would think would require quite the powerful hardware to play back.

it's for this reason that i would think that whatever the bit number in the video dac refers to, it isn't colour depth... plus, what would be the point of a dac that only displays (?) an image halfway between 8 and 16 bits?

[Lexman]

I think Ralph of Alpha DVD could probably assist with the second part Matt. I will forward this thread on to him.

Lex

[John Kotches]

I am not an expert on this topic... but here goes.

You need the 54MHz decoding rate to handle Progressive Scan output, as you're outputting twice the pixels per frame.

12bit encoding is to ensure that your 10bit data decodes are accurate out to 10bits. Most 10bit DACs are only accurate to 8 or so bits.

Regards,




---
Editor, PC/Home Theater
Secrets of Home Theater and High Fidelity
My Home Theater
NEC LT-150 specific page.

[Matt Broeska]

thanks, guys.

so to clarify on the last part re: bits... it's actually in reference to bit rate, not depth?

[Lexman]

While I get Ralph his password that he has misplaced, he wanted me to go ahead and pass on his reply to you. Quote from Ralph LaBarge follows:

Let me take a shot at answering your questions.


The increased Mhz is not related to processing power, but rather to the amount of image data that is created by the DAC (Digital to Analog Converter). DVD-Video discs have digital video data stored as MPEG-2 video. MPEG-2 video is
encoded using the 4:2:0 (YCrCb) format which means that there is twice as much luminance (Y) data as chrominance (Cr and Cb) data. The human eye is more sensitive to brightness than color, which is why the MPEG compression process throws away some color data as the first step.


When a DVD-Video player decompresses the MPEG-2 video streams it also converts it into an analog video format such as NTSC, Y/C or component video (YCrCb). This analog video is sent to the monitor connected to the DVD player. Newer model DVD players have D/A converters that have a higher level of precision than older models. This level of precision, which is usually stated as a number of bits (8 bit DAC, 10 bit DAC, 12 bit DAC, etc) is related to how many bits of information are created for the internal decompressed video signal. So a typical DVD player with an 8 bit DAC will generate a YCrCb signal in the 4:2:2
format (4 bits for luminance, and 2 bits for each color difference signal). A high end DVD player with a 12 bit DAC will generate a YCrCb signal in the 4:4:4 format.


So will a 12 bit DAC generate a "better" looking video signal than an 8 bit DAC? The answer depends on how the video is being displayed. If your DVD player is connected to your monitor using an NTSC signal, then the extra precision
generated by a 10 or 12 bit is lost before it even gets to the monitor. In this case the NTSC signal format is limiting the quality of the video signal, and improving what you send to it wont matter.


If your monitor has a component input (YCrCb) then the video generated by a 10-bit or 12-bit DAC will look a bit better since you wont be limited by the
NTSC signal format. But in general you wont be able to tell the difference in quality between a 10-bit DAC and a 12-bit DAC on a regular NTSC television monitor with component inputs.


If you have a High Definition monitor you will be able to tell the difference between a 10-bit DAC and a 12-bit DAC since a High-Def monitor can display much higher frequencies than an NTSC monitor.


So to sum it all up here is a general rule of thumb.


For an NTSC monitor connected to a DVD player using NTSC (RCA) or Y/C cables an 8-bit DAC is fine.


For an NTSC monitor connected to a DVD player using component (YCrCb) cables a 10-bit DAC is fine.


For a High Definition monitor connected to a DVD player using component (YCrCb) cables a 12-bit DAC is fine.


Hope this helps

[Matt Broeska]

i must say that's awesomely thorough. thanks so much for the info!

[rlabarge]

No problem. I work with this stuff every day so I generally know the answers to real detailed DVD questions.




Ralph LaBarge
Managing Partner, Alpha DVD
Author, "DVD Authoring & Production"
rlabarge@alphadvd.com

[Lexman]

Yeah, the words have never flowed so smooth as I said them there, lol. Great job Ralph, thanks.

Lex