Dolby Demystified
We get a lot of questions about audio processing and Dolby E and wanted to share some of the most requested info with you.
1. Dolby guardband – How to prevent playout errors
Decode or playout errors can be caused by misaligned guardband.
As Dolby E encoded audio uses a frame-based structure, the frame rate of the audio must match the video when both are wrapped into a single file. The position of the frames of audio and frames of video within the file must be corrected aligned for playback. Dolby specify a “guard band”, which is a small null band around the ‘top’ of the audio within the frame. This allows error-free cut-style editing and video switching. However note that if the guard band at the top of the frame is increased too much, then the Dolby E data for the frame will not fit into the available space within the frame and would continue into the start of the next frame. This would cause an error in subsequent decode or playout applications.
Make sure the tools you use prevent errors by providing guardband measurement and auto correction, including frame advance/retard.
Dolby Demystified: An Insider’s Guide.
2. Flexibility is key with Dolby E encoding and decoding
One challenge with Dolby E encoding and decoding is having to deal with a plethora of file formats; MOV, WAV, MXF, to name a few.
Another need to consider is support for different program configurations; two stereos, four monos, four stereos, stereo and 5.1, eight monos, the list goes on.
Look for flexible solutions that support multiple file formats, program configurations, and frame rates. Make sure the tools you choose support all frame rates, multiple file formats, and any program configuration supported by Dolby.
Choose a tool that lets you seamlessly incorporate Dolby encode and decode into your automated workflows, too.
3. Wondering about bit rate and Dolby E?
Lot’s of people ask us about bit rate and Dolby E, so we thought we’d share this brief overview with you.
Dolby E can be encoded as 16 bit or 20 bit words. When 16 bit encoding is chosen it is only possible to encode four or six tracks of audio. 20 bit word length is needed if you wish to encode the maximum of eight tracks of audio.
This has an impact on the file type that will be used to contain the Dolby E encoded data. File types such as MXF or MOV usually have audio tracks for 16 bit words or for 24 bit words. It is not possible to include Dolby E encoded audio with a 20 bit word length into files that are created for 16 bit audio, and files with 24 bit word length would be needed to carry Dolby E audio encoded as 20 bits. Note however that conversely, 16 bit Dolby E can be placed inside media files containing 24 bit audio with no limitation.
Make sure the tools you use can encode Dolby E data in either 16 or 20 bit format, and at any selected framerate.
4. Dolby + Loudness: What’s the best way to comply with regulations?
Complying with Loudness regulations can be a feat, especially when you are also dealing with Dolby processing. Requirements vary around the globe. When you’re looking for solutions to meet Loudness regulations, consider some of these issues:
- Can you measure Loudness to any of the worldwide standards?
- Can the measured value be easily inserted into metadata?
- Is sound track dialog detectable?
The Loudness module within Engine permits the measurement of Loudness to any of the worldwide standards. The measured value is automatically inserted into the Dialnorm metadata value on encoding, and dialog detection is available as an option for those who need it.
