What is Dirac PRO?
Dirac Pro is a version of the Dirac family of video compression
tools, optimised for professional production and archiving applications,
especially where the emphasis is on quality and low latency (i.e.
we avoid the long delay inherent in some of the implementations
we use for broadcast or internet applications).
Typical production processes require lossless or virtually lossless
compression with low latency. Dirac has been streamlined to meet
these requirements.
Dirac Pro is designed for simplicity, efficiency and speed, and
intended for high quality applications with lower compression ratios.
Like Dirac it is an open technology, which will work on all the
major operating systems, such as Windows, Macintosh or Linux. As
it is an Open system, it is easy to import it onto a wide range
of hardware, from specialist signal processors to application-specific
LSI circuits.
Dirac Pro is capable of being used in post production at resolutions
up to 4K with a base layer plus enhancement system, allowing very
high quality proxy workflows.
Typical applications may be
- lossless or visually lossless compression for archives,
- mezzanine compression for re-use of existing equipment, such
as 1080P 50Hz carried in a 1080I 25 Hz channel
- and low latency compression for live video links.
We can use SD infrastructure to route HD signals by compressing
1.5 GBit/s HDSDI links into 270 MBit/s SDI or SDTI. Likewise, compressing
HDSDI signals to be carried on Gigabit Ethernet (at circa 600 MBit/s)
would also allow HD working on cheap network infrastructure. DiracPRO
introduces minimal artefacts at these levels of compression.
Features
Dirac Pro will support the following techical features, required
by professional end-users:
- Intra-frame only (forward and backward prediction modes are
also available if required)
- 10 bit 4:2:2
- No subsampling
- Lossless or Visually lossless compression
- Low latency on encode/decode
- Robust over multiple passes
- Ease of transport (can use a range of transport standards including
MPEG-2 and SDTI)
- Low complexity for decoding
- Open Specification
- Multiple vendor
- Support for multiple HD image formats and frame rates.
Both Dirac and Dirac Pro are Open Technologies, and the Dirac
software source code is licensed under the Mozilla Public License
Version
1.1.
The technology of Dirac PRO
The main difference between Dirac and DiracPRO is in the treatment
of the final process in compression - the arithmetic coding. Arithmetic
coding is processing intensive and introduces delay. These are
features that are undesirable in high end production work. The
arithmetic coding produces most efficiency savings with highly
compressed material. There is little benefit to be gained with
the low compression used in top-end production. DiracPRO therefore
omits the arithmetic coding.
Applications
There are two specialised applications we have identified as prime
targets for DiracPRO.
The first is low-delay compression for live links. This uses a
special choice of motion compensation options, avoiding the delay
that the most powerful option provides. With low delay, the system
can be used for wireless links, within or outside the studio.
The second specialised application is a low compression option
designed to deliver nearly lossless coding. This lets us deliver
1080 progressive formats over infrastructure designed for 1080
interlaced.
High-end production is rapidly migrating to high quality 1080
P50/60. But this format requires a higher data rate than the existing
1.5 GBit/s HDSDI infrastructure. The DiracPRO profile supports
the transport of these high quality images over conventional high
definition infrastructure. This is an evolution of work on Mezzanine
coding (aka SMPTE VC-2) which originally used DCT as the transform.
Now we are embracing the concept within DiracPRO and using wavelets
as the prime compression tool. It is also suitable for quality
coding of video for 270 MBit/s links.
Further applications may be lossless or visually lossless compression
for archives or mezzanine compression for reuse of existing equipment,
such as 1080P 50 Hz carried in a 1080I 25 Hz channel.
For existing standard definition links, compressing 1.5 GBit/s
HDSDI links into 270 MBit/s SDI or SDTI would facilitate the use
of standard infrastructure for routing HD signals. Likewise, compressing
HDSDI signals to be carried on Gigabit Ethernet (at circa 600 MBit/s)
would also allow high definition working on a cheaper network infrastructure.
DiracPRO gives excellent quality at these levels of compression. |
