HDMI decoder
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- HillPi04
- Posts: 5
- Joined: Fri Jan 01, 2021 5:30 am
HDMI decoder
I'll try to make this as short and to the point as possible. I'm attempting my own handheld emulating project. I'm looking through available parts here on adafruit. I keep coming across projects that utilize a HDMI screen, but they have the 40pin ribbon cable going into a decoder and also the HDMI is plugged into the screen and the pi. Can someone shed some light on this for me? I guess my question is, what is the HDMI decoder used for if HDMI is an output of the pi already and second, when should one be used? Thanks in advance!
- adafruit_support_mike
- Posts: 67454
- Joined: Thu Feb 11, 2010 2:51 pm
Re: HDMI decoder
LCDs don't actually use HDMI. HDMI is a data protocol between video sources like cameras or computers, and output devices like TVs and projectors.
LCDs themselves have a driver chip bonded to the glass. It can be more than an inch long, and has connections to transparent conductor traces for every row and column on the screen. Those chips have their own data protocol and signal rules.. high-speed/high-fidelity connections use Low Voltage Differential Signaling (LVDS) for maximum reliability at the highest possible bitrate. Other displays use parallel data pins for the red, green, and blue pixel components. Others do all their communication through a regular SPI bus.
A TV's job is to convert HDMI input to signals the display driver will accept.
Our TFP401 breakout is one example of that conversion:
https://www.adafruit.com/product/2218
It accepts HDMI input and generates signals that tell a dot-clock display what to do. 'Dot clock' is a common display format based on old TV signals: the display driver moves to the next pixel in each row every time the dot-clock pulses, it advances to the next row every time the row clock pulses, and returns to the top of the display every time the screen clock pulses.
The 40-pin raw displays we carry speak the dot-clock protocol and have pinouts that match the ones at the TFP401's connector.
The RA8875 also generates dot-clock signals, but takes its input from an SPI dada connection.
LCDs themselves have a driver chip bonded to the glass. It can be more than an inch long, and has connections to transparent conductor traces for every row and column on the screen. Those chips have their own data protocol and signal rules.. high-speed/high-fidelity connections use Low Voltage Differential Signaling (LVDS) for maximum reliability at the highest possible bitrate. Other displays use parallel data pins for the red, green, and blue pixel components. Others do all their communication through a regular SPI bus.
A TV's job is to convert HDMI input to signals the display driver will accept.
Our TFP401 breakout is one example of that conversion:
https://www.adafruit.com/product/2218
It accepts HDMI input and generates signals that tell a dot-clock display what to do. 'Dot clock' is a common display format based on old TV signals: the display driver moves to the next pixel in each row every time the dot-clock pulses, it advances to the next row every time the row clock pulses, and returns to the top of the display every time the screen clock pulses.
The 40-pin raw displays we carry speak the dot-clock protocol and have pinouts that match the ones at the TFP401's connector.
The RA8875 also generates dot-clock signals, but takes its input from an SPI dada connection.
- Mikexex
- Posts: 1
- Joined: Thu Sep 23, 2021 8:40 pm
Re: HDMI decoder
I take issue with that statement. It is wrong to use the TFP401 and claim it can decode HDMI signals. It can't.adafruit_support_mike wrote:
A TV's job is to convert HDMI input to signals the display driver will accept.
Our TFP401 breakout is one example of that conversion:
https://www.adafruit.com/product/2218
What it can do is decode DVI signals, which although uses the same signalling protocol, does not have an audio channel.
It's a real shame there is no audio capability of your module.
Please be positive and constructive with your questions and comments.