Wednesday, November 25, 2009

Beware of your DVI's blanking width!

You think you know a certain signal standard but then some nuance of interpretation jumps up and bites you on the backside! Here are excerpts from an email conversation, mine are in italics, the manufacturer's in bold.

....we have a pair of VS-66HDMIs that pass signals fine at resolutions sub 1920x1200 but when you hit that resolution the output flickers and won't lock. Taking the router out of the circuit but using the same cables allows the monitor to lock to the signal fine. The two sources are; Apple MacPro with nVidia GForce GT120 graphics card Apple G5 with ATI Radio 9600 Pro graphics card. Suffice to say we've done the usual powering down etc and tried different cables in case something is on the hairy edge of spec but with different sources that seems unlikely.

Thanks for your e-mail, the details of which I sent off to our R&D people. I have now had a response, in which they said;

"When we mention 1900x1200 we mean narrow blanking. With regular 1900x1200 blanking the bandwidth is higher than the chipset capabilities."

When I asked if the VS-66HDCP matrix (6 x 6 DVI matrix) was any different the answer came back;

"They are the same and use the same chipset. You may be able to set the PC's output to narrow blanking."

So I guess the next question is whether you can adjust the PC’s to provide narrow blanking.

OK, thanks for looking into it xxxx. I had a chat with Apple Engineering late yesterday (we’re a re-seller) and they say that no stock graphics card that has shipped with either a G5 or MacPro in the last five years supports narrow DVI blanking at 1920x1200! In the case of nVidia cards that setting is exposed in the PC driver (I checked on a couple of Windows machines and that is indeed the case – although the installed default was standard rather than narrow blanking) but there is no way to get to it with OS-X.

It seems you should change your advertising to read ‘doesn’t support Macs at hi-res’ or ‘not for use with Macs & monitors greater than 23” display’ something like that. Given that an awful lot of people in the creative industries use Apple computers this isn’t an unusual requirement of a product and if someone reads the copy “Up to UXGA, 1920x1200, 1080p.” You’d forgive them for assuming that it will work with their Apple computer running at 1920x1200.

It puts us in a spot as we now have to source another pair of switchers to sort out our customer.

So there you go - I'm pinning my hopes on the Gefen equivelent. They seem to recognise that a lot of people in the TV industry use Macs!


Tech'Ed said...

Hi Phil - how did it work out with the Gefen unit? Or did Kramer tell you the truth, and the issue is with Mac?

Phil Crawley said...

Gefen was the same - it seems that they both use the same chipset and neither support narrow blanking with single-link DVI at 1920x1200.
I still lay the blame with Kramer and Gefen - it is true that *most* PC graphics card drivers can be forced into wide-blanking it is still the case that the specification allows for either and so if you're manufacturing third-party DVI equipment you should support the standard in it's entirety and not some subset in the hope that you can get away with it.
In fact the whole issue caused us to look further afield and we've started getting into Lightware products. They are much more complete that both Kramer and Gefen for DVI & HDMI but cost more. As ever you get what you pay for.

Tech'Ed said...

Sorry Phil, I think that you are missing the point. The DVI 1.0 standard lays down a maximum bandwidth of 1.65Gbps. 1920 x 1200 with anything less than reduced blanking requires a Dual Link chipset. Therefore Gefen and Kramer appear to have implemented DVI 1.0 correctly in their equipment. I found this blog quite instructive on the subject,931-6.html

Phil Crawley said...

Yes - you're right by the strict letter of the spec, but Kramer and Gefen market their equipment to the creative industries where MacPro workstations are very prevalent. Now - 1920x1200 comes out of a single-link output (i.e. it uses only four TMDS lanes - nine pins rather than dual-link's 24 pins) and so by selling a device as being "1920x1200" compatible they really should mention " long as you don't expect your Mac to work with it and a 24" monitor" or some such.
The truth is that graphics cards have been driving 1.72Gbps over single-link for several years and Kramer/Gefen et al. do the industry a disservice by hiding behind the 1.0 standard.