Electronic Projects, forums and more.

Go Back   Electronic Circuits Projects Diagrams Free > Electronics Categories > Micro Controllers
PAL vs. NTSC PAL vs. NTSC
Register Blogs Calendar Search Today's Posts Mark Forums Read


Micro Controllers Discuss all aspects of micro controllers - building them, coding them, etc. All controllers are welcome - PIC, BASIC, Z8 Encore!, etc.

Reply
Page 2 of 2 < 1 2
 
Thread Tools Display Modes
Old 6th June 2008, 10:24 PM   (permalink)
Default
The Stanford document (the other NTSC thread) uses a 63.5 usec horizontal line with a 4.7 usec 'sync' pulse, a 5.9 usec 'back porch', 51.5 usecs of video data, and a 1.4 usec 'front porch'. That works out to a 15,748 KHz frequency. Those timings also work out perfectly for the 100 nsec Tcy when using a 40 MHz crystal on an 18F' device.

There must be some degree of variation that allows the TV to work with different horizontal frequencies?

Mike
Mike, K8LH is offline   Reply With Quote
Old 7th June 2008, 02:31 AM   (permalink)
Default
Quote:
There must be some degree of variation that allows the TV to work with different horizontal frequencies?

Mike
The same Stanford document you mentioned explicitly suggests as much. I think getting in the ballpark of 63.5 microseconds per scan line is sufficient, the ballpark being within 2 or 3 microseconds. The important thing seems to be making sure that whatever time for that is the closest you can manage, that it's exactly consistent from scan line to scan line.
Hank Fletcher is offline   Reply With Quote
Old 7th June 2008, 02:36 AM   (permalink)
Default
Point is crystals come in many frequencies, why would you use something like 20MHz when you could use something that's a better fit for the job at hand? Try digikey for some common frequencies.
__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com
blueroomelectronics is offline   Reply With Quote
Old 7th June 2008, 02:38 AM   (permalink)
Default
I have that same mini TV. It sucks lol.
Krumlink is offline   Reply With Quote
Old 7th June 2008, 03:24 AM   (permalink)
Default
14.31818MHz is exactly 910 times the NTSC horizontal frequency, and exactly 4 times the NTSC color subcarrier frequency. You can generate fully interlaced color NTSC if you start with this frequency. Obviously, if you don't need color, you can start with any frequency that will divide down to 15.734kHz and also will give you enough resolution to get reasonably accurate pulse widths for horizontal sync, vertical sync, and equalizing pulses.
__________________
Ron
Roff is offline   Reply With Quote
Old 7th June 2008, 06:08 AM   (permalink)
Default
Quote:
Originally Posted by blueroomelectronics
Point is crystals come in many frequencies, why would you use something like 20MHz when you could use something that's a better fit for the job at hand?
I get your point, but I guess I was figuring that if I was trying to do this with a 16F88 or such, and the fastest it'll run is 20MHz, that I'd get a better resolution with a faster chip? I mean, it might not divide neatly into the NTSC standard, but given the suggested leeway for horizontal sync times, that might not be an issue. Personally, I'm not worried about the NTSC police telling me it doesn't conform exactly to the standard - I'm just a tinkerer trying to make the most of what I've got.

Yes, as long as I'm studying and learning I appreciate the value in conforming to the standards and products normally associated with certain applications, but on the other end of things, I think there's some value to trying to explore what can be done with the tools most readily at the disposal of mcu novices. If that can be an entry point to those novices choosing a direction to their liking that's faster, bigger, etc, so be it.
Hank Fletcher is offline   Reply With Quote
Old 7th June 2008, 06:12 AM   (permalink)
Default
Crystal, oscillator, resonator: what's the difference, what's the trade-off?
Hank Fletcher is offline   Reply With Quote
Old 7th June 2008, 01:33 PM   (permalink)
Default
Quote:
Originally Posted by blueroomelectronics View Post
Point is crystals come in many frequencies, why would you use something like 20MHz when you could use something that's a better fit for the job at hand? Try digikey for some common frequencies.
I guess I don't understand why you think the odd freqeuncy is "better fit" for the job when when anyone should be able to generate a 63.5 usec scan line with almost any Fosc.

I cobbled together an isochronous loop below to work out timings. It seems I should be able to paint 120 lines of 256 dots (200 nsecs per dot at 40 MHz) or, probably better, 240 lines of 128 dots (400 nsecs per dot at 20 MHz). ** this is not working code, just a timing exercise...

Mike
Code:
;
;  assuming RA0 connected to resistor(s) to produce the 0.0/0.3v
;  level and RB0 connected to resistor(s) to produce the 1.0v
;  level.
;
;  the first 2 delays would be used to stuff the 32 byte work
;  buffer.  The 588 cycles available in each of the 20 vertical
;  sync' lines would be used for updating the video buffer.
;
frame
        movlw   242             ;
        movwf   hctr            ; set horizontal line counter
hline
        bcf     LATA,0          ; 00.0 usecs - set 0.0v 'sync' level
        DelayCy(47)             ; "sync"
        bsf     LATA,0          ; 04.7 usecs - set 0.3v 'blk' level
        DelayCy(57)             ; "back porch"
B00     movf    POSTINC0,W      ;
        movwf   LATB            ; 10.6 usecs
        nop                     ;
        rrcf    LATB,F          ; 10.8 usecs
        nop                     ;
        rrcf    LATB,F          ; 11.0 usecs
        nop                     ;
        rrcf    LATB,F          ; 11.2 usecs
        nop                     ;
        rrcf    LATB,F          ; 11.4 usecs
        nop                     ;
        rrcf    LATB,F          ; 11.6 usecs
        nop                     ;
        rrcf    LATB,F          ; 11.8 usecs
        nop                     ;
        rrcf    LATB,F          ; 12.0 usecs
;~~~~~~~
B01..B30 ; same code for bytes 01..30
;~~~~~~~
B31     movf    POSTINC0,W      ;
        movwf   LATB            ; 60.2 usecs
        nop                     ;
        rrcf    LATB,F          ; 60.4 usecs
        nop                     ;
        rrcf    LATB,F          ; 60.6 usecs
        nop                     ;
        rrcf    LATB,F          ; 60.8 usecs
        nop                     ;
        rrcf    LATB,F          ; 61.0 usecs
        nop                     ;
        rrcf    LATB,F          ; 61.2 usecs
        nop                     ;
        rrcf    LATB,F          ; 61.4 usecs
        nop                     ;
        rrcf    LATB,F          ; 61.6 usecs
;~~~~~~~
        nop                     ; 61.7 usecs
        nop                     ; 61.8 usecs
        movlw   20              ; 61.9 usecs
        movwf   vctr            ; 62.0 usecs
        clrf    LATB            ; 62.1 usecs - set 0.3v 'blk' level
        DelayCy(14-3)           ; "front porch"
        decfsz  hctr,F          ; 63.2 usecs - 242 lines?
        bra     hline           ; 63.3 usecs
        nop                     ; 63.4 usecs
vsyn1
        bsf     LATA,0          ; set 0.3v level
        DelayCy(47-1)           ; inverted "sync"
        bcf     LATA,0          ; set 0.0v level
        DelayCy(588-4)          ;
        decfsz  vctr,F          ; all 20 vsync lines?
        bra     vsyn1           ; no, branch, else
;
;  the vsyn1 code needs to be modified to jump to "frame" (a
;  new frame) with the correct timing...
;
Mike, K8LH is offline   Reply With Quote
Old 7th June 2008, 02:01 PM   (permalink)
Default
Quote:
Originally Posted by Hank Fletcher View Post
Crystal, oscillator, resonator: what's the difference, what's the trade-off?
The tradeoff is accuracy. A crystal is more accurate than a simple resonator.
__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com
blueroomelectronics is offline   Reply With Quote
Old 7th June 2008, 02:53 PM   (permalink)
Default
Quote:
The tradeoff is accuracy. A crystal is more accurate than a simple resonator.
And the only advantage to using an oscillator over a crystal is that they're capable of higher frequencies. Given their use for PICs, however, I see the crystals go up to 80MHz from digikey, which appears to far exceed the maximum frequency for any of the PICs I've been using. I can't see the point in using an oscillator for a PIC, given oscillators appear to be more expensive and cumbersome, but maybe I'm missing something here?
Last edited by Hank Fletcher; 7th June 2008 at 02:54 PM.
Hank Fletcher is offline   Reply With Quote
Old 7th June 2008, 03:32 PM   (permalink)
Default
An advantage of a resonator may be built in caps, cheap, and shockproof (crystals are not happy about shocks)

The 14.31818 will get divided by 4 by the PIC, that will make it less than ideal for a 910 division (TMR2)
So the magic crystal frequency will be something that multiplies by 62937.0549Hz the closer to 20MHz the more breathing room you'll have.
__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com
blueroomelectronics is offline   Reply With Quote
Reply
Page 2 of 2 < 1 2

Bookmarks

« 20Mhz PIC & HD44780-clone | PIC clock cycles, instructions, and crystal frequency? »
Thread Tools
Display Modes
Linear Mode Linear Mode


Similar Threads
Thread Thread Starter Forum Replies Latest
Ntsc TKS Micro Controllers 5 1st June 2006 08:44 PM
Convert an NTSC in-car TV mcncy Electronic Projects Design/Ideas/Reviews 4 21st March 2004 09:47 AM
NTSC-PAL conversion apakhira Electronic Projects Design/Ideas/Reviews 0 25th February 2004 05:04 PM
NTSC to PAL CONVERTER thesim Electronic Projects Design/Ideas/Reviews 9 13th February 2004 03:32 PM
NTSC to PAL convertor circuit (NTSC=>RGB; RGB=>PAL) apakhira Electronic Projects Design/Ideas/Reviews 2 30th January 2004 05:05 AM



All times are GMT. The time now is 01:37 AM.

Electronic Projects Circuits Diagrams - Archive - Top

Electronic Circuits  |  Electronics Wiki
Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2008, Jelsoft Enterprises Ltd.