Electronics > Projects, Designs, and Technical Stuff

Recording HDMI

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TheUnnamedNewbie:
Your friend probably did not get ripped off (ripped off in the sense that someone sold him a cheap product for a lot of money. I'm not gonna comment on if he needs such a device - clarification later).


--- Quote from: Jan Audio on June 21, 2020, 11:10:52 am ---What chip do you need to make a simple HDMI recorder to SD card ?
I bet a PIC32 in DIP can not handle it as usual.

--- End quote ---

It is impossible, even for the best engineers on the planet, to use any off-the-shelf general purpose microcontroller to do HDMI capture (unless you get some special one that happens to have HDMI transceivers in it though I don't know if those exist).

Why? It's quite simple really: like most video interfaces, HDMI is uncompressed. The 'transmitting device' uncompresses the video stream and then sends this over the cable. As a result, HDMI has quite high data-rates, together with other video interfaces, is likely (by a good margin) the fastest digital link through a cable in a consumer house. My 10 Gbit/s Ethernet can't hold a candle to the near 50 Gbit/s that the HDMI2.1 ports in my TV can handle.

HDMI1 has three channels. Each of these sends data at a maximum rate of 1.65 GBit/s. (for a total signal rate of 4.95 GBit/s, however, the net data-rate is only 3.96 GBit/s due to the 8b10b coding). There is no way any microcontroller or even most microprocessors that don't have specific HDMI transceivers can handle this. This is why most capture cards use either a HDMI-specific IC, or an FPGA (which has high-speed SERDES) to handle this.

This also brings us to the second problem:


--- Quote from: Jan Audio on June 21, 2020, 11:10:52 am ---What chip do you need to make a simple HDMI recorder to SD card ?

--- End quote ---

Lets have a look at SD cards: The second-fastest spec is the 'UHS-III' transferrate, which (on paper) can do 624 MB/s. Spoiler: there exist no cards that can actually sustain that datarate. V90 cards are the fastest, and that '90' is for '90 MByte/s sustained write speed' (which is 720 Mbit/s). So, we can't just take the data of the HDMI stream, and somehow dump it into SD card. The SD card just can't keep up. This is why high-end cinema cameras record to SSDs. (and even then they use (possibly lossless) compression first).
 
Most capture cards fall into three categories:
1) Low-quality compression based on some HDMI receiver IC and then a microprocessor or an ASIC (not that familiar with this market, to be honest but with streaming becoming so common, there is really a new market for these ASICs)
2) CPU-based compression: just act as an HDMI translation layer, stream it to a internal bus like PCIe, thunderbolt, and then use the CPU of the system to compress (I think a lot of modern CPUs/GPUs also come with H264/H265 hardware compression integrated).
3) High-quality compression based on an ASIC or FPGA implementation. These do high-bit-rate H264 or H265 or similar compression on-the-go. This is what a lot of the higher-end cards do that are used in media. These solutions range from hundreds of dollars to tens-of-thousands for entire live-broadcast multi-camera high-speed recording setups that can record 20 or more 4k60fps streams at the same time.

The gap between type 1 and type 3 is closing (or rather, you can now find affordable options that can do good-enough-for-most-applications compression). Broadcast will still demand more of their cards, and higher control over the compression.

I have to say that I can find HDMI recorders in the local larger computer/electronics stores.

Jan Audio:
Understood, thanks all.
So many pixels, top technology.
Starting to have more respect for my TV.

Larryc001:
Would something like this not work?

https://www.amazon.ca/AVerMedia-EzRecorder-Definition-Recording-ER130/dp/B00LAP3GC8

DaJMasta:

--- Quote from: Jan Audio on June 22, 2020, 02:43:39 pm ---Understood, thanks all.
So many pixels, top technology.
Starting to have more respect for my TV.

--- End quote ---

It is really impressive what they can cram into a specialized single chip solution these days.  If you look at the early 1080p+ TV designs you sometimes find stacks of FPGAs on multiple processing boards just to keep up!

Nerull:

--- Quote ---I think he got ripped off, it is not allowed to sell a device that can record HDMI or am i wrong ?
There are no HDMI video recorders in any shop.
--- End quote ---

I picked one up at Best Buy a couple months ago. Couldn't find a webcam within 500 miles and the next best option was to hook up my Canon 5D. Always have the best video quality in the meeting....

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