At midnight on March 20, 2017, Dolby's last relevant patent on Dolby Digital expired.

Standard-Essential Patents Declared by Dolby

ETSI TS 102 366: Digital Audio Compression (AC-3, Enhanced AC-3) Standard

Expiration Patent Title
n/a US6246345 (E-AC-3 only) Using gain-adaptive quantization and non-uniform symbol lengths for improved audio coding
2017-03-19 US5890106 Analysis-/synthesis-filtering system with efficient oddly-stacked singleband filter bank using time-domain aliasing cancellation
2015-06-07 US5633981 Method and apparatus for adjusting dynamic range and gain in an encoder/decoder for multidimensional sound fields
2014-05-20 US5632003 Computationally efficient adaptive bit allocation for coding method and apparatus
2014-04-22 US5623577 Computationally efficient adaptive bit allocation for encoding method and apparatus with allowance for decoder spectral distortions
2013-12-10 US5583962 Encoder/decoder for multidimensional sound fields
2013-12-03 US5581653 Low bit-rate high-resolution spectral envelope coding for audio encoder and decoder
2012-12-26 US5479562 Method and apparatus for encoding and decoding audio information
2012-10-13 US5291557 Adaptive rematrixing of matrixed audio signals
2012-02-28 US5394473 Adaptive-block-length, adaptive-transforn, and adaptive-window transform coder, decoder, and encoder/decoder for high-quality audio
2011-10-18 US5357594 Encoding and decoding using specially designed pairs of analysis and synthesis windows
2011-06-21 US5274740 Decoder for variable number of channel presentation of multidimensional sound fields
2011-03-22 US5297236 Low computational-complexity digital filter bank for encoder, decoder, and encoder/decoder
2010-10-15 US5235671 Dynamic bit allocation subband excited transform coding method and apparatus
2010-05-12 US5752225 Method and apparatus for split-band encoding and split-band decoding of audio information using adaptive bit allocation to adjacent subbands
2009-12-29 US5109417 Low bit rate transform coder, decoder, and encoder/decoder for high-quality audio
2008-08-29 US4914701 Method and apparatus for encoding speech
2005-12-06 US4790016 Adaptive method and apparatus for coding speech

Source: Dolby IPR for TS 102 366 (ETSI)

What is Dolby Digital (AC-3)?

AC-3 is a compressed digital audio format like MP3. It made its public debut in 1992. AC-3 has become the most common format for audio in film and television.

AC-3 supports up to 5.1 surround sound.

Has Dolby acknowledged that their last AC-3 patents are expiring?

In Dolby's 2005 S-1 filing with the SEC, Dolby acknowledged that AC3 patents expire in 2017.

"Patents relating to our Dolby Digital technologies expire between 2008 and 2017."

Dolby has only two AC-3 patents with 2017 expiration dates: US6449368 (expiring on 2017-03-14) and US5890106 (expiring on 2017-03-19).

Dolby provides a list of standard-essential AC-3/E-AC-3 patents to ETSI as part of the DVB standardization process. The last active AC-3 patent in this list expires on 2017-03-19. Dolby has not amended this list since 2004.

How does a public standard stay "patented" for longer than 20 years?

Dolby engages in legally dubious strategies known as "evergreening" to abuse the patent system and extend their licensing monopoly for AC-3. Some of these tactics include:

Users

I'm a user. Why should I care?

You have probably paid many AC-3 license fees over the years. AC-3 license fees are part of the cost of TVs, game consoles, and other AV equipment sold in the last 25 years.

You may have had problems playing certain video files on your computers and mobile devices. In 2012, Dolby started threatening the authors of various video player apps, forcing developers to remove their apps, remove support for AC-3, or increase prices.

Developers

When would I need to decode AC-3?

When would I need to encode AC-3?

Hopefully never! In practice, there are a few cases:

Is AC-3 any good? Should I use it for new stuff?

No, of course not. AC-3 is over 25 years old and horribly inefficient.

How can I safely use AC-3 in my app?

There are multiple open source implementations of the AC-3 encoder and decoder. Some general guidance:

The following open source libraries implement AC-3:

Will Dolby pretend that they still have patent rights to AC-3?

Probably!

In 2012, Dolby bought a vague submarine patent from Panasonic, US6339757 ("Bit allocation method for digital audio signals"). The patent describes an optimization for audio encoders. (… for MiniDisc, using the Sony ATRAC codec.) Dolby has started claiming that Dolby Digital and all of their own products are already implementing this patent.

The good news:

How can I make sure this never happens again?