Software Dolby-B|C Compatible Compander

  DDi Codec is a precise digital equivalent of the classical Dolby-B/C noise reduction system for analog audio tape. It is the world's first dedicated software capable of decoding or encoding Dolby-C formatted audio in digital domain, along with full backward compatibility to Dolby-B. Powered by the proprietary DSP engine, the decoding/encoding can be performed in realtime pace, which allows instant monitoring and control of quality.
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Deeper optimal result can be achieved by using the fine-tuning tools that come with the software, as specified below:

  • The Reference Vernier is the primary tool for aligning Dolby Reference Level in an progressive and interactive manner. It supports either objective calibration (with Dolby Tone) or subjective estimation (without Dolby Tone).

  • The Azimuth and Gap-Loss tools can compensate minor imperfection of an analog tape player used for digitizing audio tapes.

  • The Play Trim is another tool for compensating weakened signal on aged pre-recorded tapes.

  • The EQ Converter offers digital translation between IEC 120μs and 70μs tape EQ schemes — useful if such an options is not available on a given tape player/recorder.

  • The built-in tone generator offers 400Hz 0dB signal at Dolby Reference Level for easy calibration.
All these tools can work independently from the Dolby-B/C codec, which can also be applied for refining non-Dolby formatted audio.

DDi Codec is a stand-alone GUI app (without relying on any other DAW host). It allows the computer to perform as a outboard Dolby-B/C NR unit in one of the following four modes:

Input Output Comment
Audi-In File a digitizer/recorder, with B/C codec
File Audio-Out an audio file player, with B/C codec
File File an off-line B/C processor, with batch capability
Audi-In Audio-Out an on-line B/C processor, with slight delay

Currently supported audio file formats are:
WAV, FLAC, AIFF: up to 96k/24bit.
MP3, AAC, M4A: up to 48k/VBR.
* Excluding some proprietary variants having the common file extensions, or additional format-normalization may be required.

A VST plugin is also available after the app has been installed. However, it is currently in experimantal stage and requires the full application being installed on the same computer.

  Audio Demo #1 (Symphony)
Audio Demo #2 (Electronica)
Audio Demo #3 (Vocal)

Why It Matters?

Many valuable audio contents are only available on specific media of their era, for example, cassette tapes in '70s ~ '90s. They deserve extended live in digital age and being easily appreciated via contemporary media. When being digitized, the cassette tapes with Dolby-B/C encoded contents may come with extra advantage, which is lower noise — in theory. But the reality often goes to the opposite due to the risk of Dolby-B/C tracking errors at decoding end, aka: mistracking. At least three factors may cause the unexpected mistracking: 1) regular service (calibration) of both recorder and player; 2) tape degradation over time; and 3) inconsistencies among Dolby-B/C implementations. People who are not fully aware of or not properly equipped to control the tracking errors may suffer inferior results, or even give up.

Fortunately, Dolby-B/C mistracking is easily audible to human ears — it doesn't require lab instruments to identify. It typically manifests as excessive muffled sound after being decoded. However, fixing the problem is another story, which is much harder than detecting it, because almost all Dolby-B/C systems in cassette players were not adjustable by users. In other words, they were not designed flexible enough for processing off-standard inputs. So, in order to properly restore the original audio, an user-adjustable Dolby-B/C decoder would be needed, but such an adjustable decoder was hardly available in the real world for decades.

Many people do believe that Dolby-B/C decoding can be done in digital domain. This is true but not because digital is any good at implementing Dolby-B/C system. Actually, Dolby-B/C system is a rather unique analog audio companding system that is very challenging to emulate in digital domain by a discrete algorithm. The long absence of such a algorithm already spoke for the challenge. But once it is done, the advantage is also significant because of the way a computer works. Mitigating Dolby-B/C tracking errors is not a one-shot deal. It is a trial-and-error decoding workflow which replies heavily on user's hearing feedback. A computer with modern DSP engine and graphic user interface (GUI) is an ideal platform that allows the job being done in an interactive, progressive and cost-efficient way.


What quality can be expected from DDi Codec software vs. analog Dolby-B/C hardware?

Analog hardware was the native form of Dolby-B/C system. As of today, using analog Dolby-B/C hardware is still the best choice in theory. However, no new analog Dolby-B/C hardware is in production today. In fact, vast majority of the analog Dolby-B/C units found today were produced decades ago and missing regular maintenance as required. They might drift out of the design specification more than one can anticipate, causing Dolby mis-tracking and dull sound. The hardware here refers to tape recorders/players with built-in Dolby-B/C processor and external (outboard) Dolby-B/C analog companding units.

Yes, it is possible to restore an aged Dolby-B/C hardware unit back to its specification, although it requires qualified service, expertise and price. But this is only one side of the coin. Mistracking error can still occur even with a well calibrated Dolby-B/C hardware unit used for decoding purpose. The question is on the tapes. Magnetic signal on a given tape may degrade over time, or it was simply encoded by an off calibration encoder in the first place. Thus, the Dolby-B/C encoded audio on a given tape may already be off standard. So, a standard sticking decoder is no longer perfect. It becomes a common feature request for a decoder to be able to adapt to all the conditions at the end of the entire chain. This was hardly viable for a typical analog Dolby-B/C hardware that was designed inflexible out of the factory. A new tune-able decoder with adjustable parameters and fine-tuning pre-processor will be needed.

Interchangeability is another issue found between different hardware implementations of Dolby-B/C system, which was largely overlooked. Dolby Labs had licensed various manufactures to implement Dolby-B/C patents in various practical forms. Since the original Dolby-B/C prototype was given by discrete circuits, almost all the manufactures had foreseen the need to embody the system in low-cost integrated circuits. However, the original discrete circuits were based on mixed types of semiconductor, which was rather challenging to be exactly imitated with a single micro chip (at that time). Creative workarounds and progressive improvements were the common R&D footprints in making the dedicated Dolby NR ICs. This inevitably introduced cross-generation inconsistencies and cross-manufacture(brand) inconsistencies, although they all aimed to closely matching the same root standard. Consequently, devices that used different Dolby-B/C ICs made in different years by different manufactures inherited such inconsistencies and presented mistracking errors when cross-processing Dolby-B/C formatted audio. This type of mistracking is non-serviceable because it can't be completely eliminated by after market re-calibration. In other words, calibration on single point at 0dB can not kill all errors on other levels. However, the overall errors can still be made less significant by purposely and temporarily calibrating the decoder slightly off the 0dB. But again, it requires a customizable decoder that offers such a flexible control to users.

Since digital age, there seemed to be a new easy solution — retooling Dolby-B/C system in software. While the wish is fantastic, the fact is not as easy as it sounded. Dolby-B/C system has very unique characteristics of its own, which is not found in any conventional DSP library in general. Attempting to simulate Dolby-B/C by customizing a linear EQ plugin is a simplistic, inaccurate and amateurish idea.

Design of a perfect software Dolby-B/C codec relies on exact algorithmic transformation of the original analog Dolby-B/C system, including the specific filter network topology and the JFET-based variable shunt, along with the challenge to transform the instantaneous non-linear feedback loop into a discrete-time algorithm. Obstructed by these challenges, the demand of a software Dolby-B/C codec has never been met for decades since the beginning of digital age. Some former efforts attempted to work around these challenges by using feedforward instead of feedback, which compromised sound quality and computation efficiency because it required up-sampling and then down-sampling the audio so to reduce the predictive errors. Other attempts used fixed-band side chain with variable gain instead of sliding-band side chain, which produced a Dolby-like sound effect rather than an exact codec. Up until recently, the challenges have been addressed in a new research project with intensive mathematical derivations. The output demonstrated a new path to precisely reproduce the unique non-linear sliding-band characteristics of Dolby-B/C system in digital domain. A new dedicated DSP algorithm has been developed with computation efficiency even suitable for low consumption embedded CPUs. Based on the new DSP engine, DDi Codec has been developed as a dedicated software application that strives for the state-of-the-art digital codec for Dolby-C (with full backward compatibility with Dolby-B) which is not only sounding correct but also measured correct.

DDi Codec is a software product intended to provide a digital alternative to those who are not fully confident/satisfied with the aged analog Dolby-B/C hardware but are willing to accept digital help. Compared to analog hardware, The software app will never drift off design specification over time but also offers the needed flexibility in working with imperfect tapes and players, getting the most out of them. The software app will operate as a typical external (outboard) Dolby-B/C compatible compander with additional convenient calibration options, which unleashes great potential to achieve optimum results in working with the classic NR system. As a software solution, it is for the first time to allow Dolby-B/C decoding/encoding tasks being able to cross-benefit from the gifted digital advantages, such as: lossless fidelity, zero new noise, mathematical accuracy, wide dynamic range, low cost, no wear and no maintenance. The software makes it possible to post-decoding existing digital archives (audio files) which are still Dolby-B/C encoded, removing the hard pre-emphasis artifacts which was otherwise impossible without re-digitizing the original analog tapes. The file-to-file decoding can be done much faster than using a real-time-only hardware. For studios/publishers, the software allows direct integrating Dolby-B/C encoding task into the digital mastering workflow and feed it to a digital mass-duplication system, avoiding unnecessary losses in analog processing, and offering alternative licensing solution too.

Since the official program for granting Dolby-B/C license has been long closed, it is impossible for DDi Codec to claim any official proof of quality. Use of this software is still at user's own risk. However, the processor's performances have been measured and summarized in the following data sheets, along with online audio demos. Hope they can help in making an easy, informative and objective pre-evaluation.


The measured self-encoding then self-decoding curves have been omitted from here because they are almost perfect flat lines.

Finally, while DDi Codec is intended to be a precise digital equivalent of the analog Dolby-B/C system, it may not incidentally enhance "analog taste" of the digital audio which is rather a subjective matter.

Audio Demo #1 (Symphony)
Audio Demo #2 (Electronica)
Audio Demo #3 (Vocal)

Dolby-B and C are the tape noise reduction systems developed by Dolby Laboratories.

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See also:
DxII Codec (for dbx-II/Disc)
DxI Codec (for dbx-I/Disc)
NAK T-100 (Audio Analyzer)