Lost in Transmission, Part One

Transmission-Figure1-770.png

A few years ago, after giving a presentation on the “Future of Display” at LDI, I made a narrowly controversial statement. I was asked, “When will professional video presentation systems used in live events be completely wireless?” My response? Never, that’s when. Once the cries of “heresy!” died down, I explained that my belief is that the bandwidth and reliability demands of high-end video display systems will always exceed the capability of wireless systems and that hardware, not waves traveling through the air, will be needed, at least during my lifetime.

Whether you agree with my prediction or not, it is safe to say that today’s video presentation systems require wire and other bits of hardware to transmit signals from the output of one device to the input of another. Also, now that network-based video transmission systems have begun to penetrate the marketplace, a primer on the various transmission systems seems like good idea.

NOTE: The system described are primarily used for transporting video data amongst video devices. Specific systems that can accommodate other types of data such as control or audio are noted.

The Basics

The subject of video signal transmission is comprised of two intertwined elements: signal format and transmission format. Signal format is determined by the devices being connected, while “transmission” is based on the choice of system that will accommodate the signal format and pass it from one device to another.

For example, if the video output card in the media server you are using has an HDMI connector on it, a transmission system that supports HDMI must be used. That transmission system might be fiber or copper-based, or it might be a networked video system, but it must accommodate an HDMI format signal.

To illustrate, Table 1 describes the commonly used signal formats:

Name

Capability

Connector Type

EDID*

HDCP**

Other

RGB/HV

Older analog format limited to 2048x1536 @ 85 Hz

HD15 or 5x BNC

No

No

No

DVI

Older analog/digital format limited to 1920x1200 @60 Hz (single-link)

DVI

Yes

Optional

No

HDMI 2.0

Current digital audio and video format supporting signals up to 4K @ 60Hz & HDR

HDMI-types

Yes

Yes

8x Audio, Ethernet†

HDMI 2.1

Introduced 2017 with support of 8K @ 50Hz. Higher bandwidth in certain configurations.

HDMI-types

Yes

Yes

32x Audio,

DisplayPort 1.4

Packet-based protocol with support up to 8K @ 30Hz, higher with DSC.

DP-types

Yes

Yes

8x Audio, USB

HD-SDI

SMPTE standard video interface supporting 720p, 1080i HD video.

Single BNC

No

No

16x Audio, CC, Timecode

3g SDI

SMPTE standard video interface supporting HD & 4K @ 30Hz video.

Single BNC

No

No

16x Audio, CC, Timecode

12g SDI

SMPTE standard video interface supporting HD & 4K @ 60Hz video.

Single BNC

No

No

16x Audio, CC, Timecode

Transmission Systems

It requires expertise and experience to select the best transmission method for any particular situation and that expertise can’t be bestowed in the context of a single article. However, a survey of what is available and their general characteristics may be helpful to those who work with the types of video devices commonly used in live entertainment, such as media servers, LED video displays, and projectors.

In live entertainment when we discuss transmission systems, we are talking specifically about connecting devices across a significant distance, such as from a front-of-house position to the stage or from a “video village” located backstage to video projectors flown in an overhead rig. To connect devices a short distance apart, say co-located in an equipment rack, the transmission system is typically the short cable that connects one device to another with, perhaps, an adapter of some type (example: DP to HDMI).

However, when the cable distance exceeds 50', a transmission system should be employed, and for very high-resolution signals, 10' is often the limitation for standard cables. For example, you may be able to use a 50' HDMI cable to connect a 1080p HD signal, but a 2160p 4K signal may have issues using that same 50' cable. In those cases, a system that accepts the original signal, converts it to a transmission format, and then re-converts it back to the original is required.

The simplest long-distance video signal transmission system uses a transmitter and receiver (transceiver) set connected over twisted-pair copper cable, which can be standard Ethernet Category 5 or 6, shielded or unshielded, depending on the requirements of the specific transceiver. These transceivers are essentially balancing transformers that provide a robust enough signal to support cable runs of 1,000' or more.

Twisted-pair transceivers are sold by a huge number of manufacturers and have a vast range of features, including audio and data transmission. All manner of signal formats are supported, including DVI, DP, and HDMI, and they can handle signals as high as 4K @ 60Hz. Interference rejection is fair to good.

They also provide great value as the transceivers are not very expensive and the cable, even in road-worthy touring form, is cheap. For these systems to work properly, attention must be paid to the bandwidth requirements of the originating signal and the exact specification of the twisted-pair cable. Some systems require a proprietary form of cable to meet spec, while others are more flexible. Check the manufacturer’s specification carefully when planning to use twisted-pair transceivers, particularly any requirements for wall power at transmit or receive sites.

References

Stay tuned for Part Two, which covers HDBase-T, SDI, and Video Over IP.

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