Streamed Vs. Linear Broadcast Architectures

One of the important differences is the way colorimetry is specified. The range of colors is constrained in traditional broadcasting because illegal colors cannot be reliably transported through the systems. This is due to legacy limitations existing long after analog video has been deprecated. Streaming services are not limited because they never used analog video.

Streaming services are wholly file-based. Instead, a streaming server can deliver the contents of a file made by the production workflow. This file would have already been prepared for streaming and no further compression is needed. Live feeds are presented as virtual files.

On-air head-end systems multiplex several channels of uncompressed material into a transport stream. Live material is piped direct from the studio gallery and files are injected as if they were live feeds.

Where The Standards Apply

Here is a typical broadcast system transmitting linear content. This might originate from an external source or as pre-recorded programmes played out to the transmission system.

The architecture is quite different for a streaming Video-On-Demand (VOD) service. The playout and transmission are replaced by a request driven Content Delivery Network (CDN).

The applicable ingest standards are described in delivery specifications based on the individual broadcaster's needs.

Delivery Specifications

The delivery specifications define file formats, metadata and supporting media for external production companies when they supply content.

Netflix has a delivery specification geared towards streaming services. Other streaming services will likewise have their own specifications. This is necessary when content is arriving from many diverse sources and facilitates the ingest by automated systems.

Public service on-air broadcasters in the UK have formed the Digital Production Partnership (DPP) to pool their expertise. They are:

• BBC
• BT Sport
• Channel 4
• Channel 5
• ITV
• Sky
• STV
• TG4

The DPP has a liaison with the North American Broadcast Association (NABA).  The DPP also works closely with other standards bodies and has a membership of over 500 media, production and broadcasting companies. You can also join as an individual.

The collaboration with NABA has yielded a common file format based on IMF for the exchange of program material. This is published by SMPTE as TSP 2121.

The public-service broadcasters are all regulated in the UK by Ofcom. Each country has its own Regulatory Authority. Streaming services are not currently regulated in the same way although this may change. The Ofcom broadcasting code is incorporated into the delivery specifications for DPP organizations and adds these requirements:

• Photosensitive Epilepsy guidelines.
• Younger viewers should be protected from seeing or hearing harmful content.
• Equality and discrimination must be taken into account to support disabled viewers.
• Strict taste and decency guidelines are enforced and verified by inspecting compliance recordings.

Delivery specifications also cover metadata and other ancillary items such as sub-title texts, supporting still photographs and poster artworks which are included in the manifest.

Netflix and DPP both indicate that Apple ProRes coded video in an iTunes compatible container is also acceptable.

SMPTE ST 2067 IMF Files

The Interoperable Master Format (IMF) is the foundation for a lot of other file formats which are described as 'IMF Applications' in ST 2067. Application 2 extended (part 21) is specified by Netflix and the DPP as a potential delivery format.

The ST 2067 standard has many parts, not all of which are applicable in every situation. The parts are grouped into categories according to what they describe:

Here is a list of the currently released parts with their revision dates:

IMF Application DPP - TSP 2121

The SMPTE TSP 2121 standard is the basis of the DPP file format. This is an IMF Application and developed with NABA in the USA. Refer to other standards when necessary. ST 2067 will be useful as will the colorimetry specifications from the ITU.

Colorimetry Specifications

This is a complex subject. Human visual perception naturally compensates for color shifts, different dynamic ranges and brightness levels.

The colorimetry standards describe transfer functions that map the input brightness levels to the output. When the output color gamut cannot include all the colors in the input, the transfer function ensures the image is perceived correctly under all viewing conditions.

Video color legalization in traditional broadcasting limits the dynamic range of pixel color values. Streaming services are not affected by this and can accommodate the full range of values.

Basic transfer functions are implemented with Gamma correction shown here with positive and negative values alongside the unmodified linear transform.

Dolby Laboratories developed a more sophisticated technique for displaying High Dynamic Range (HDR) content. This is called the Perceptual Quantizer (PQ). It greatly enhances the detail in the darker regions of the image. This is a simplified example with the positive Gamma curve to show the difference.

The BBC collaborated with NHK on an alternative approach called Hybrid Log-Gamma (HLG). This applies gamma correction to the lower range of values and logarithmic correction to the upper. It doesn't increase detail in the dark region as much as PQ. The mid-range values are increased more than the basic Gamma transform but less than PQ. The join between the two curves must be implemented carefully to avoid discontinuities that would introduce contouring (posterization) artefacts in the output images. The positive Gamma curve is included for comparison.

Here are some relevant colorimetry standards:

* The version dates for the Dolby Vision metadata specs are approximate and based on available documentation.

DVB Standards For On-air Broadcasting

The Digital Video Broadcasting organization (DVB) manages a collection of international open-standards that describe how to transmit digital TV services on-air. DVB is a not-for-profit consortium sponsored by the broadcasters. Their standards are grouped into these categories:

The DVB standards have been the foundation for digital TV deployments worldwide and describe how TV signals can be transmitted in different ways:

The most familiar DVB standards describe how digital TV services are structured for transmission. The second-generation variants add high definition support:

Multiple program streams are packaged into a bundle. On DTT services, these are called Multiplexes and on DSat they are described as Transponders. The bundle also includes metadata for Service Information (SI) to populate Electronic Program Guides (EPG).

Conclusion

The SMPTE IMF file format is an industry standard preferred by many organizations that need to share content. Whilst there are alternatives that may be technically superior, some of them are proprietary.

The Dolby Vision™️ work on colorimetry has facilitated the wide adoption of High Dynamic Range (HDR) content which enhances the Ultra High Definition (UHD) viewing experience to a very high standard. There are open and non-proprietary alternatives if you need them.

The creation of world-wide standards such as TSP 2121 move us closer to content that can be deployed in many countries with little or no post-processing required.

If this level of collaboration continues, the future does look very encouraging but it will depend on agreed standards to make it all work.

These Appendix articles contain additional information you may find useful: