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Code Issues Releases Wiki Activity

[TUTORIALS] 15khz TV signal generation

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This commit is contained in:
Till Harbaum
2016-01-18 21:21:19 +01:00
parent 16e6c6298d
commit 514dc81908
1 changed files with 21 additions and 25 deletions
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46
tutorials/soc/readme.md
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@@ -641,39 +641,36 @@ Files required on SD card:
![Lesson 11: TV](lesson11/lesson11.png)
All tutorial so far have used the VGA output to generate a [VGA
All tutorials so far have used the VGA output to generate a [VGA
compatible video
signal](https://en.wikipedia.org/wiki/Video_Graphics_Array). But one
of the big advantages of an FPGA is it's ability to generate all kinds
of signals. The VGA output is thus not limited to VGA signals. Its
three analog outputs could actually even be used to generate three
audio channels or to transmit and RF signal.
audio channels or to transmit an RF signal.
But a more obvious usage is the generation of TV signals. Even today
many TVs accept standard definition (SD) TV signals in form of a [RGB component video signal](https://en.wikipedia.org/wiki/Component_video#RGB_analog_component_video)
via e.g. their [SCART](https://en.wikipedia.org/wiki/SCART)
inputs. Many machines of the homecomputer era generated such signals
so it makes sense to generate these with the MIST as well for the
perfect retro game experience.
perfect retro game experience using a TV as the display.
The main timing difference between TV and VGA signals lies in the line
frequency and the number of lines displayed per image. While VGA
screens display at least 31000 lines per second a TV signal only
consists of 15625 lines per second. Furthermore the European PAL TV
signal only draws 50 frames per second (50 Hz) while VGA draws at
least 56 frames per second (56 Hz). American NTSC TV draws 60 frames
per second (60 Hz).
screens display at least 31000 lines per second (31 kHz) a TV signal only
consists of 15625 lines per second (15 kHz). Furthermore the [European PAL TV
signal](https://en.wikipedia.org/wiki/PAL) only draws 50 frames per second (50 Hz) while VGA expects a signal to carry at least 56 frames per second (56 Hz). [American NTSC TVs](https://en.wikipedia.org/wiki/NTSC) draw 60 frames per second (60 Hz).
A TV input thus doesn't cope with VGA signals and vice versa. The
easiest method to make a TV signal somehow VGA compliant is to draw
every line twice. This is called scan doubling and makes an NTSC TV
signal compatible with most VGA screens. Scan doubling a 50 Hz PAL
signal results in a VGA signal with 50Hz frame refresh rate. Many VGA
screens cope with this but not all. Especially TVs often don't accept
these not-100%-VGA signals on their VGA inputs. Most cores contain
such scan doublers since the machines they implement orignally were
connected to TVs and thus generate TV video signals while the primary
use case of the MIST board is with VGA screens.
Thus a SCART TV input usually doesn't cope with VGA signals and vice
versa. The easiest method to make a TV signal somehow VGA compliant is
to draw every line twice. This is called scan doubling and makes a
60 Hz NTSC TV signal compatible with most VGA screens. Scan doubling a 50 Hz
PAL signal results in a VGA signal with 50Hz frame refresh rate. Many
VGA screens cope with this but not all. Especially TVs often don't
accept these not-100%-VGA signals on their VGA inputs. Most cores
contain such scan doublers since the machines they implement were meant to be connected to TVs and thus generate TV video signals while the
primary use case of the MIST board is with VGA screens.
It is possible to connect the VGA output of the MIST with a TVs RGBS
input like e.g. its SCART connector. Since the TV expects TV signals
@@ -703,8 +700,8 @@ This tutorial includes a retro video unit
[video.v](https://github.com/mist-devel/mist-board/raw/master/tutorials/soc/lesson11/video.v). This
can either output a PAL like video signal or an NTSC like video
signal. The mode can be changed via the OSD. The resulting video
signal is the converted into a VGA signal by
[scandoubler.v](https://github.com/mist-devel/mist-board/raw/master/tutorials/soc/lesson11/scandoubler.v). This
signal is then converted into a VGA signal by
[scandoubler.v](https://github.com/mist-devel/mist-board/raw/master/tutorials/soc/lesson11/scandoubler.v). The
scan doubler can be disabled via the [`scandoubler_disable` option in
the `mist.ini`
file](https://github.com/mist-devel/mist-board/wiki/DocIni#scandoubler_disable)
@@ -717,11 +714,10 @@ The resulting core will generate a 50 or 60 Hz VGA signal without a
TV compatible PAL (50 Hz) or NTSC (60 Hz) signal for direct connection
to a TVs RGBS input. Many old CRTs also support this.
Links: - [MIST Scart
cable](https://github.com/mist-devel/mist-board/wiki/ScartCable) -
[Disabling scandoubler in MIST .ini
file](https://github.com/mist-devel/mist-board/wiki/DocIni#scandoubler_disable)
- [Buy a cable](http://lotharek.pl/product.php?pid=168)
Links:
- [MIST Scart cable](https://github.com/mist-devel/mist-board/wiki/ScartCable)
- [Disabling scandoubler in MIST .ini file](https://github.com/mist-devel/mist-board/wiki/DocIni#scandoubler_disable)
- [Buy a cable](http://lotharek.pl/product.php?pid=168)
Files required on SD card:
- [`soc.rbf`](https://github.com/mist-devel/mist-board/raw/master/tutorials/soc/lesson11/soc.rbf) renamed to `core.rbf`