[Coco] RGB to VGA boards, test equipment needed

[Gene Heskett]

On Sunday 06 September 2015 23:59:11 Barry Nelson wrote:

> What these people recommending expensive oscilloscopes are forgetting
> is that you want to look at the video SYNC pulses. The frequency of
> the horizontal sync from a color computer is only 15khz, which is in
> the audio frequency range. The vertical sync is even slower. That
> means, if all you want to see is the sync pulse even a cheap audio
> scope should give you a pretty good idea of what the sync pulse is
> like. The video pixels will be a fuzzy blur, but that shouldn't
> matter.

You are forgetting that the sync pulse you want to see has a maximum 
duration of 4.7 microseconds when looking at an old NTSC signal wannabe.  
Its fall and rise times are 120 nanoseconds when filtered for the 4.2 
Mhz bandwidth of an NTSC signal.  So for an good look, you'd need at 
least 20 samples during that nominally 5 microseconds. The reciprocal of 
that is 4 megasamples a second.  Thats not software doable, needs an 
expensive hardware sampler the software just displays the results of.

> That means that these should work (at least somewhat):
>
> http://makezine.com/projects/sound-card-oscilloscope/
> https://www.zeitnitz.eu/scope_en
> http://www.zelscope.com
>
> Since your sound card can sample at up to a 44khz wave that means it
> should get at least 3 samples per period of a 15khz wave, it won't be
> pretty, but you should be able to at least see the H sync  pulse.

I doubt that.  This is not a sine wave, but a well defined square wave 
with components at frequencies as high as 4 or 5 megahertz.

> You 
> won't see the pulse shape but you should be able to see a spike

no, it will look more like a noisy sine wave, no doubt with aliasing 
artifacts out the wazoo.

> and 
> measure the amplitude

Only fleetingly and occasionally.

> and the timing.

Not at all without a very deep memory to store samples, and many minutes 
for an auto-correlator to reconstruct the waveform of a 30 second or 
longer capture.

We got pix back from the pluto flyby using such techniques, but the 
auto-correlator itself cost us taxpayers millions.


> The vertical sync should show up  
> in more detail as it is slower.
>
> If you want to see the shape of the H sync pulse you will need a
> faster sample rate.
>
> Something like this one:
>
> http://www.ebay.com/itm/Hantek-6022BE-PC-Based-USB-Digital-Storag-Osci
>lloscope-2-Channels-20MHz-48MSa-s-/271265189899?hash=item3f28aa500b
>
> Bandwidth: 20MHz; Sample Rate: 48MSa/s;
>
> Or this one
>
> http://www.ebay.com/itm/SainSmart-DDS120-Pro-20MHz-PC-Based-USB-Digita
>l-Storage-Portable-Oscilloscope-/300992154516?talgo=origal&tfrom=271265
>189899&tpos=unknow&ttype=price&ssPageName=STRK:MESE:IT
>
> Bandwidth: 	20MHZ
> Maximum sampling rate: 	50M/S

These should be adequate, but you didn't quote the asking price.

> On Sep 6, 2015, at 10:22 PM, coco-request at maltedmedia.com wrote:
> > Date: Sun, 6 Sep 2015 19:30:35 -0500 (CDT)
> > From: "tim franklinlabs.com" <tim at franklinlabs.com>
> > To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> > Subject: Re: [Coco] RGB to VGA boards, test equipment needed
> > Message-ID:
> > 	<693991787.2888.1441585835674.JavaMail.vpopmail at atl4oxapp101.mgt.ho
> >sting.qts.netsol.com>
> >
> > Content-Type: text/plain; charset="utf-8"
> >
> >   You might want to consider something like this... It's 100Mhz.
> > There are many of these from many different manufacturers. I can't
> > speak for this one personally but showing this for reference...
> >  
> > [1]http://www.circuitspecialists.com/usb-oscilloscope-dso-2250.html
> >
> >     On September 6, 2015 at 6:43 PM Kandur <k at qdv.pw> wrote:
> >     Thanks Gene.
> >     What I really need these toys for is, to check the video
> > signal's sync pulses,
> >     amplitude, shape and polarity.
> >     Most of the time I would use these for to check audio signals.
> >     Do these have X and Y direct inputs, like the analog scopes do?
> >     I don't have room for large boxes anymore, like this
> >     Tektronix 7623A 100 Mhz Storage scope, I used to have in my
> > shop. http://tinyurl.com/nbhla8n
> >     Kandur
> >
> >     Sunday, September 6, 2015, 3:20:13 PM, you wrote:
> >> On Sunday 06 September 2015 16:19:14 Kandur wrote:
> >>> Would these pocket oscilloscopes do for checking video signals?
> >>> If yes, wich one is better? Are there any others under $100?
> >>> http://tinyurl.com/p9jx2ec
> >>> http://tinyurl.com/ommymqs
> >>
> >> eye.
> >> above will, and because they are analog, often under priced on
> >
> >     ebay.
> >
> >> I have one of these:
> >
> >    
> > <http://www.ebay.com/itm/Hitachi-V-1065A-Portable-Two-Channel-Analog
> > -Oscilloscope-100MHz-R-Type-w-Handle-/181840967212?hash=item2a5690f6
> > 2c>
> >
> >> contains a good part you can use.
> >> Cheers, Gene Heskett
> >> "There are four boxes to be used in defense of liberty:
> >> -Ed Howdershelt (Author)
> >> Genes Web page <http://geneslinuxbox.net:6309/gene>
> >
> >     --
> >     Coco mailing list
> >     Coco at maltedmedia.com
> >     https://pairlist5.pair.net/mailman/listinfo/coco


Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>