Advanced Digital Video (Part 2)

Usually when you're troubleshooting digital video, it’s usually not because of the actual video data,

but the ancillary video tech like these that come along with it.

These provide communications between the source, sink (display/endpoint device) and repeater devices.

This is a very valuable slide. I would take a screen shot of this if I were you.

At a very high level, this shows the handshake process that takes place when you connect two HDMI devices.

If you learn nothing else during this presentation besides this one slide it will have been time well spent

and should really help you during your troubleshooting sessions with digital video.

First our source sends a +5V,

a sink interprets that as a connected source and in turn sends a Hot Plug Detect response.

You often hear about “hot plugging” a source or sink which means removing and reseating the HDMI cable,

and basically restarting the whole HMDI handshake process.

Once the source receives the HPD, it will then request EDID, and then the sink will respond with the EDID data.

At this point one of two things will happen: if the source is HDCP compliant it will send an HDCP request.

If the source is not HDCP compliant, then the source will start delivering unencrypted video via TMDS.

If the source is HDCP capable and has sent an HDCP request then it will perform the HDCP negotiation

and if everything goes well as planned it will begin to encrypt that video data as needed.

The video timing sent by the source is determined through reading the sink’s EDID

and sending video data compatible with the sink EDID.

Let’s talk about DDC, or Display Data Channel.

This is an older technology that uses an I2C technology bus.

An I2C bus consists of three wires: SDA (which is data), SCL (which is clock), and a logic "high" DC pull voltage.

For the DDC, the logic "high" voltage is specified to be +5V.

The DDC specifications define a +5V supply connection for the source to provide the power

to a display's EDID circuitry so that communication can be enabled, even if the display is powered off.

Here's where you see the HDMI connector where theDDC is. Pin 15 is for the SCL

and pin 16 is for the SDA.

EDID (or extended display identification data) allows the sink to tell the source what it can and can't do.

A display basically says “I support these resolutions and frame rates” …

and then the source will send appropriate video data based on the EDID it reads over the DDC.

The EDID carries the preferred resolution, which is typically what the source will output.

EDID started out with VGA, and then moved on to DVI, HDMI, and Display Port.

It was developed by VESA, and it's transmitted today over at the E-DDC, but we still just call it DDC.

There are EDID standards, structure, and data blocks.

When speaking about EDID is good to keep these in mind to help from getting confused.

Here’s a quick look at the EDID standards available today.

With structure, we still mainly use 1.3 but some 1.4 structures are starting to appear.

Blocks are what actually contains EDID information.

The primary data block 0 is always required, and one or more extension blocks might exists as well.

Most commonly found extension block is the CTA-861,

which supports advanced capabilities of consumer devices that incorporate HDMI.

Data block 0 is where we find the preferred timing and other characteristics of the sink

and if there are extension blocks that's where we'd look.

Now there are many extension blocks supported in EDID but the CTA-861 extension block,

which is shown here, is almost always available in most HDMI sinks.

It contains additional sink characteristics not already defined in the primary block 0.

As we have learned, EDID is read from sink, display or projector.

It identifies connection type, supported resolutions, its preferred resolution,

supported audio and color space information.

Based on reading EDID,

a source or repeater device knows what to send and will format its video data appropriately.

Our Q-SYS NV Series also does some intelligent EDID filtering so connected sources

don’t send any video data that is not supported on this device.

This ensures that if the device EDID is copied and used on any HDMI input in the Q-SYS Design

it will be 100% compatible with NV Series devices.

For example, we accept up to eight channels of LPCM audio on any HDMI input but we don't currently

accept Dolby or DTS encoded audio.

In this case, we remove the support for Dolby,

DTS and any other forms of bitstream audio and only list up to 8 channels of LPCM audio.

Next is DRM, which stands for Digital Rights Management.

And more specifically, when we talk about DRM we're usually talking about HDCP.

HDCP protects the intellectual property of the creators, making it harder to copy or steal that content.

For better or for worse, HDCP is here to stay, so let’s do our best to understand it.

This video encryption process uses three steps: authentication, encryption and key renewal.

A source device initiates an authorization handshake protocol using keys and encrypted messages

to determine if a connected device is authorized and able to receive encrypted content.

All downstream devices must be authorized for encrypted content.

Once the source device has determined that all downstream devices

are HDCP compliant and the maximum number of devices has not been violated,

encrypted content is then sent to all downstream devices.

Encryption keys are periodically updated while devices are connected and sending content.

Secret keys are used during the encryption process,

so if those secret keys are compromised the whole system can be circumvented.

All compromised keys are revoked and the revoked list compared against any downstream devices

to verify only legitimate public keys are used.

Let's be honest, HDCP is often the root of significant issues in transporting digital video.

Sometimes when it is not needed, it might be worth turning it off.

That's why we added that feature to the NV-32-H.

On the HDMI inputs of each of these devices, you're able to disable HDCP.

Recall, HDCP doesn't just apply to HDMI but it is also valid on other interfaces.

It is transmitted over the DDC, same as EDID.

HDCP was designed to prevent content from being played or recorded on unauthorized devices.

Therefore, all authorized devices must obtain a license, pay annual fee and submit to certain conditions.

Here is a brief timeline of HDCP.

As you can see, more interfaces began to adopt the protocol as time goes by.

But … you might be asking "why do you care about these versions?"

Well, for starters, version 2.2 is not backwards compatible with version 1.4.

HDCP 1.4 is typical for HD content and HDCP 2.2 is typical for 4K content.

Since they are not backwards compatible between 2.2 and 1.4,

manufacturers need to build devices that support both 1.4 and 2.2.

There are three types of devices in the HDCP link: the source, the sink and the repeater.

Now, the source and the sink are pretty self-explanatory.

However the Repeater accepts content, decrypts it, and then re-encrypts and retransmits the data.

Repeaters have HDMI inputs and outputs.

Which is another reason why we care about HDCP versions - it's the device count limitation

As you can see, HDCP 1.4 allows more devices in in the chain than 2.2. 7 levels of repeaters versus 4.

In general HDMI 1.4 is likely to be used with HDCP 1.4, and HDMI 2.0 is likely to be used with HDCP 2.2.

However, some HDMI 1.4 (HD) devices may support HDCP 1.4 or 2.2

and some HDMI 2.0 (4K) devices may support HDCP 1.4 or 2.2.

Remember this slide? Well, it’s back!

Let's recall that the source will send +5V, the sink will respond to the HPD event,

the source requests the EDID, and the EDID is sent to the source,

then the HDCP is then negotiate and the video data is sent encrypted or unencrypted.

CEC is not necessarily part of the handshake or hotplug process,

but it is an additional bidirectional ancillary data channel for device control over HDMI.

Okay, let's step away from your computer, let your eyes uncross, go get some fresh air,

and we'll come back whenever you're ready.