Brief review of the BenQ BL912 19" LCD monitor

Arc/RPCs, peripherals, RISCOS operating system & ARM kit eg GP2x, BeagleBoard
Post Reply
User avatar
IanB
Posts: 386
Joined: Sun Sep 04, 2011 7:28 pm
Location: South Wales
Contact:

Brief review of the BenQ BL912 19" LCD monitor

Post by IanB » Sun Jan 10, 2016 10:31 pm

Most monitors are now 16:9 but it looks like some companies are still producing 1280x1024 17" and 19" 5:4 monitors and the BenQ BL912 is one of them.
I decided to order one of them after reading a comment in this thread about it's capability to display normal 15Khz TV modes:
viewtopic.php?f=41&t=10560
which led to this thread on Amibay:
http://www.amibay.com/showthread.php?72 ... for-amigas
There is also a 17" version that works at 15Khz: BL702A

I've tried it with an A5000 and the result is that it does work and you can switch between VGA, SVGA and 15Khz TV modes but there are some issues:

The main problem is that the 15 Khz modes have poor sampling of the video resulting in uneven pixel widths. This is because all digital video systems assume a sampling rate of 13.5 Mhz on 15Khz modes and the dot clock on Acorn systems is 16Mhz in 640x256 modes.
This is a lot less noticeable in 320x256 modes which are used by most games so if you intend to run the desktop in VGA or SVGA mode and only switch to the 15Khz modes for games then it is probably not going to be a problem. If you want to use 15Khz modes for everything then it may not be so good but there may be a hardware & software workaround to fix that. SCART inputs on TVs also sample at 13.5Mhz but they have antialiasing filters which mean that they don't have uneven pixels but instead have a soft picture which loses details in 640x256 modes. (These modes have a video bandwidth of 8Mhz but they are filtered to 6Mhz by the SCART input).

Here are some off screen pictures: (The moire patterns and barrel distortion are artefacts of the camera and not present on the screen)

MODE 12 (640x256) desktop:
Mode12.jpg
Notice the uneven lines of 'M' and 'W' and also the vertical bars on the HD icons due to the 13.5 Mhz sampling.

MODE 27 (640X480) desktop:
MODE27.jpg
MODE 31 (800x600) desktop:
MODE31.jpg
MODE13 (320x256) test showing slight variation in pixel widths but not as noticeable as 640x256 modes:
MODE13.jpg
Some games in 320x256:
PAC1.jpg
PAC2.jpg
ZARCH.jpg
ELITE.jpg
The monitor does support interlaced video but it uses simple 'bob' deinterlacing which results in shimmering simlar to a CRT on any interlaced video.
However as the the Archimedes doesn't use interlaced video this is not a problem and in fact can be an advantage as TV modes are simply scaled to full screen without using any sophisticated deinterlacing which can cause strange motion artefacts. (This can be noticeable on SCART inputs of newer TVs)

I decided to experiment and created a 15Khz version of MODE 12 that had a 13.5 Mhz dot clock instead of 16Mhz on a RISC PC as that is easy with !MakeModes. I then loaded a screensave of the a5000 desktop into that as the RISC PC desktop is not directly comparable:
13.5MhzTest.jpg
As you can see, this one is perfect with the lines of 'M' and 'W' all even and the vertical bars on the HD icons also even. The picture is also a little wider and makes more use of the screen

It should be possible to make an addon similar to the existing clock boards (which add extra crystals for VGA and SVGA modes) to use an additional crystal of 20.25Mhz to produce a 13.5 Mhz dot clock to replace 16 Mhz (for 640 pixel modes) and 6.75Mhz dot clock to replace 8Mhz (for 320 pixel modes). Together with an appropriate mode module, all TV modes would then conform to the digital TV standard. This would even help SCART inputs as the lower clock speed would reduce the bandwidth of the video and result in a sharper albeit wider picture on an LCD TV.

It seems that quite a few new monitors have undocumented support for 15Khz modes on their VGA inputs. This is mainly because 576i is part of the HDMI standard so the chipsets that handle all of this have that built in now but it is almost certain that they will all have the same sampling issues as above because they all assume a sample clock of 13.5Mhz.

To sum up, there is no perfect solution, all options seem to have their problems although maybe this type of monitor together with a crystal modification might be one of the best.

Zarchos
Posts: 2355
Joined: Sun May 19, 2013 8:19 am
Location: FRANCE

Re: Brief review of the BenQ BL912 19" LCD monitor

Post by Zarchos » Mon Jan 11, 2016 10:10 am

IanB wrote: It should be possible to make an addon similar to the existing clock boards (which add extra crystals for VGA and SVGA modes) to use an additional crystal of 20.25Mhz to produce a 13.5 Mhz dot clock to replace 16 Mhz (for 640 pixel modes) and 6.75Mhz dot clock to replace 8Mhz (for 320 pixel modes). Together with an appropriate mode module, all TV modes would then conform to the digital TV standard. This would even help SCART inputs as the lower clock speed would reduce the bandwidth of the video and result in a sharper albeit wider picture on an LCD TV.
Thanks a lot for your review and these very helpful explanations, to squeeze every ounce of power off the guts of an Archie.

User avatar
sbadger
Posts: 391
Joined: Mon Mar 25, 2013 1:12 pm
Location: Farnham, Surrey
Contact:

Re: Brief review of the BenQ BL912 19" LCD monitor

Post by sbadger » Tue Jan 12, 2016 2:00 pm

you maybe able to improved the sampling artifacts at 15khz, most monitors have an option to adjust this, or just plain not sample.

On this one

http://www.manualslib.com/manual/732134 ... =29#manual

seems these have a displaymode option to limit geometric distortion.?
So many projects, so little time...

User avatar
IanB
Posts: 386
Joined: Sun Sep 04, 2011 7:28 pm
Location: South Wales
Contact:

Re: Brief review of the BenQ BL912 19" LCD monitor

Post by IanB » Tue Jan 12, 2016 11:00 pm

Most of the options like overscan are greyed out. The ones that should fix it are "clock" and "phase" but they don't have enough adjustment range. The clock allows the sample frequency to be adjusted but It doesn't seem to have the range of adjustment to get from the default of 13.5Mhz to 16Mhz.
Increasing the clock setting does result in a wider picture but it doesn't completely remove the sampling error.

I wonder if there is a service mode that can provide more adjustment.

User avatar
IanB
Posts: 386
Joined: Sun Sep 04, 2011 7:28 pm
Location: South Wales
Contact:

Re: Brief review of the BenQ BL912 19" LCD monitor

Post by IanB » Wed Jan 13, 2016 1:31 pm

I found the service mode but there was nothing in there that would have helped.
However I have managed to get a high quality picture without making any hardware changes:

I decided to try using the mode definition parameters for the 13.5Mhz mode but use the 16Mhz clock instead. This meant that the number of pixels per line was as the monitor was expecting but the frame rate would be higher (59 Hz instead of 50Hz). Much to my surprise that worked and gave a clean picture with no sampling artifacts:
mode12-59hz.jpg
Closeup showing current resolution as 720x288 @ 59Hz (640x256 + borders)
modeinfo.jpg
At the moment this is still being generated by a RISC PC so I will try and get the correct mode module working on my a5000 and see what the games look like. It would still be possible to change the crystal to get the correct frame rate but it wouldn't be essential to get a good picture.
Running at a higher frame rate may mean that games run at the wrong speed but I think that is also a problem with the current LCD game modes module as that uses modes running at 70Hz.

Post Reply