Which surround sound format delivers
better home theater sound?
This Dolby vs. DTS surround sound is a rather hotly debated issue. Many argue that DTS Digital Surround is capable of delivering better sound quality than its Dolby Digital counterpart. But is this really so?
In this article, we discuss the main differences between these formats; this should lead to better understand why this Dolby versus DTS controversy prevails among home theater enthusiasts and professionals alike.
The Dolby vs. DTS Debate
Identifying the main differencesbetween these two multi-channel sound formats
Many audiophiles and home theater enthusiasts argue that DTS surround sound formats can deliver higher quality sound than corresponding Dolby formats, with improved dynamic range, better subtle detail in audio content, and improved signal-to-noise ratio.
Partly, this reasoning arises as DTS surround sound is usually encoded at a higher data rate than Dolby Digital and its lineup of associated formats.
This reasoning is more than understandable. We are dealing with lossy compression techniques. Higher bit-rate for the same format should normally implies superior sound during playback, as less compression in the encoding should result in a better representation of the original sound source. In this Dolby vs. DTS debate, DTS people argue that they are after ‘sonic perfection, not space consumption’ – hence the use of a higher bit rate and lower compression. Instead, Dolby counteract that their codec is more efficient and therefore, it can operate at a lower bit rate. In other words, Dolby’s argument in this Dolby vs. DTS debate is that they have managed to come with a codec that can achieve higher compression levels for the same sound quality.
Dolby vs. DTS: Is it really all codec efficiency versus sound quality?
The truth is that one cannot simply draw conclusion on sound quality based on raw bit rates and compression figures alone; it also depends on how well designed are the encoding/decoding algorithms. But then…
There are relatively big differences in raw bit rates and compression levels when comparing Dolby vs. DTS sound formats, implying a too wide a difference in codec efficiency – circa 3 times as much – in favor of Dolby. Is it possible that Dolby sound formats achieve this level of codec efficiency?
The reality is that when dealing with different sound formats, the whole equation starts to get extremely complicated – especially as it is not easy to define what constitutes better sound.
This Dolby vs. DTS is a rather hotly debated issue, but in reality is it true that one is better than the other?
To understand why this Dolby vs. DTS surround sound controversy prevails, it is necessary to have an understanding of the main differences between these formats – in particular, the differences in bit-rate and compression levels as applicable to Dolby Digital and DTS Digital Surround.
And as if to further complicate this whole issue, there are also significant differences in the way these formats are implemented in moviehouse applications and in home entertainment.
Dolby vs. DTS: Bit Rate & Data Reduction
Dolby Digital and DTS Digital Surround are both 5.1 surround sound systems that encode six discrete audio channels into blocks of data in their respective Dolby or DTS bit streams.
Sound on audio CDs uses 16-bit linear PCM-encoded samples at a sampling rate of 44.1 kHz for each audio channel; this yields a data bit rate of 705 kbits/s for each channel, and a dynamic range of 96dB from the loudest sound that can be handled by the system to noise floor. Encoding 5.1 sound at CD-audio sampling rate and resolution yields a bit rate in excess of 3.5 Mbit/s.
Now, both Dolby Digital and DTS Digital Surround encoding schemes allow for an even higher sampling rate of 48 kHz at 20-bits per sample – thus yielding an even wider dynamic range between sound level extremes of approximately 120dB.
Compression – a necessary Evil!
This means that these surround sound formats would yield too much data to store, and equally important, to process in an efficient and economical manner in real time during playback. In other words, under a multi-channel scenario, compression becomes a necessary evil.
It is here that the real difference between these sound formats emerges crisp and clear. And ultimately, it is the significant difference in compression ratios and bit rates adopted by these two formats that has led to so much controversy in this Dolby vs. DTS surround sound debate.
Other Significant Differences
Compression and bit-rate are not the only differences when comparing Dolby vs. DTS formats. For example, the added rear surround channel in Dolby’s extended surround format ‘Dolby Digital EX’, is matrixed over the two left and right surrounds, rather than discrete; instead the DTS counterpart uses a discrete channel. This also explains why DTS ES (Extended Surround) can provide a more precise location for the rear-effects soundstage than the Dolby EX format.
However, this difference is not one of the main issues that are fuelling this Dolby vs. DTS debate. It is a fact that multi-channel matrixed sound cannot produce the same level of precise localization of sound effects in the surrounding soundstage –so basically there is nothing to argue about!
The real Dolby vs. DTS controversy arises because when dealing with different formats that use different coding techniques to deliver practically the same end result, it is not easy to define what constitutes better quality sound.
Data Reduction Techniques
Both Dolby Digital and DTS Digital Surround systems make use of data reduction techniques to remove redundant data in the original PCM signal, and to eliminate audio data that would result inaudible to the human ear – thus reducing the space required to store multi-channel sound.
The fundamental idea is to reduce the data size by re-coding the signal with sophisticated data reduction techniques, while allowing for more accurate audio (20-bits samples instead of 16-bits) than that supported by linear PCM used in CD-audio, without increasing the required bit rates.
The reduction in data represents the removal of objective and perceptual redundancies that are in the original PCM audio signal. The re-coding algorithms consist of a lossless coding part to remove the objective redundancies, and a lossy coding part to remove the perceptual redundancies by using psychoacoustic models. The latter are used to remove data that do not reduce the quality of acoustic perception, and therefore inaudible to the human ear.
Psychoacoustic analyses exploit an effect called ‘auditory masking’; in addition the whole process adapts the input signal to the spectral sensitivity curve of the human ear.
The effect of auditory masking describes a phenomenon in which a frequency with higher amplitude covers a frequency in a short spectral distance with lower amplitude. Thus the frequency with the lower amplitude is masked and cannot be heard; thus coding of these masked frequencies is redundant. It can be seen that frequencies above a masking frequency are easier to mask than frequencies below.
This is mainly due to the human ear frequency response. At the higher frequencies, the sensitivity of the ear is reduced. Thus higher frequencies must produce stronger amplitude to be recognized. If the amplitude of a frequency is below the perception threshold, that frequency is redundant and does not need to be encoded.
This data reduction process uses a filterbank to split the signal into a number of frequency sub-bands, while sophisticated global bit management techniques are used to calculate the ideal bit rate for each channel and sub-band while maintaining a constant global bit rate.
The above implies that when it comes to comparing Dolby vs. DTS, both make use of lossy compression techniques since the final decoded audio data can never represent a perfect image of the original source content.
Dolby vs. DTS – Bit Rates and Compression Levels
Dolby Digital compresses the resultant data for the discrete 5.1 channels of digital audio down to a maximum raw bit-rate of 640 kbits/s. Yet the 640 kbits/s limit is only supported on Blu-ray discs; HD DVD, DVD-video and DVD-Audio limits the Dolby Digital maximum bit rate to 448kbit/s, while the optical implementation of this format in theatrical applications limits the maximum bit rate even further.
In order to be able to squeeze in all relevant data, Dolby Digital applies variable compression of approximately 10 to 12:1.
DTS Digital Surround uses a maximum raw bit-rate of 1.5 Mbits/sec even though on DVD-video, this is limited to around 768 kbits/sec. At this higher bit-rate supported by the format, DTS requires substantially less compression – approx. 4:1 – than its Dolby counterpart. This means that theoretically, DTS Digital Surround has the potential to produce higher quality sound. But as stated earlier on…
In this Dolby vs. DTS comparative analysis, raw bit rates and compression levels alone cannot be taken as a direct measure of sound quality since there are other factors that come into play here – in particular, the efficiency of the coding/decoding algorithms, as well as the overheads in terms of bits required to manage the raw bit stream itself.
Difference in Moviehouse and Consumer Applications
Dolby vs. DTS: Moviehouse Applications
The real major technical difference when comparing Dolby vs. DTS sound formats arises in the way these are implemented in movie theater applications.
Dolby Digital soundtracks are optically recorded on the 35mm film strip itself using sequential data blocks placed between sprocket holes as shown in the diagram below, at a fixed reduced raw bit rate of 320kbit/s.
In addition to the optically encoded audio data, the space between the sprockets also includes the Dolby Logo.
Dolby Digital is at times referred to as Dolby SR-Digital in moviehouse applications in view that a movie with Dolby Digital soundtrack would normally still come with a conventional SR (Spectral Recording) Dolby analog soundtrack as shown in the diagram.
|35mm film strip with multiple encoded soundtracks|
With four squares of digital information per frame and 24 frames passing through the projector each second, each space between individual sprockets has to hold about 3330 bits of data; this data is read by a CCD scanner during playback, and then processed to extract the Dolby Digital (AC-3) bitstream.
The main reason for implementing both digital and analog versions on the filmstrip is twofold – as a backup in case the digital soundtrack fails, and also so that only one set of film prints would be required for cinemas with or without the ability to reproduce the digital soundtrack.
Placing audio content on the filmstrip itself has got its pros and cons. Having the Dolby Digital soundtrack direct on the film means that audio content is susceptible to physical damage due to film aging and mishandling. However, that there is no need for delivery of separate audio CD discs to theaters, thus simplifying distribution.
In the case of a DTS soundtrack, audio information is not stored direct on the film strip itself, instead it is recorded in compressed form on a standard CD-ROM media at a bit rate of 1103 kbits/s. The DTS track on the film strip consists of a modified time-code that is optically imaged on the film strip (see film strip diagram above); this is used to synchronize the projected image with the DTS surround sound audio track on the CD.
Additional information in the imprinted time-code includes identifying data to ensure that a certain film soundtrack will only run with that film.
Because of the increased storage capacity available on CD-ROM, DTS digital soundtracks need only about 4:1 adaptive compression. Though this is a lossy scheme, yet at this relatively high bit rate, there is hardly any discernible difference between the original soundtrack and the reproduced sound after decompression.
In other words, in moviehouse applications, the higher data rate afforded by the CD-ROM media means that theoretically, DTS surround sound technology is in a position to deliver better audio fidelity than that afforded by Dolby Digital movie soundtracks.
The use of separate CD-ROM media in DTS movie sound – while complicating somewhat the distribution process itself, carries a number of advantageous; these include the use of multi-lingual soundtracks without changing the film print, and equally important, CD-ROM media is less subject to damage by repeated screenings, and as such rarely requires replacement.
Dolby Digital and DTS Surround Sound in the Home Theater
Comparing Dolby vs. DTS sound in consumer applications reveals that Dolby Digital and DTS Digital Surround are closer in terms of audio performance.
When the DTS soundtrack is encoded at the highest bit-rate of 1.5 Mbits/s defined by this DTS standard, DTS claims that it is capable of achieving indistinguishable audio fidelity from its original un-coded source content.
Similar claims come from Dolby Labs, namely that when Dolby Digital is encoded at the maximum 640 kbits/s defined by this standard, it is capable of achieving similar audio perceptual transparency results, in that the listener would not be able to distinguish between the original un-coded source and the output from a coded soundtrack. Yet, in real life implementation, things are somewhat different…
The maximum raw-bit rate supported by the either format is not always supported by the respective media used in the home theater.
DVD-video format limits Dolby Digital sound tracks to a bit rate of 448 kbits/s. At this reduced bit rate, perceptual audio transparency is lost. On some DVD releases, this is even further limited to a bit rate of 384 kbits/s. In either case however, these bit rates are higher than that used in moviehouse applications.
Similarly, DVD-video limits DTS-audio to a reduced bit rate of 768 kbits/s, which is substantially lower than that used in a movie theater DTS sound installation. At these reduced bit rates, one cannot speak of perceptual audio transparency for either format, yet these represent distinct differences when comparing Dolby vs. DTS in the home theater versus the respective moviehouse application for the same format.
For a Dolby Digital sound track, a properly equipped home theater installation is capable of achieving better surround sound than its corresponding Dolby Digital audio playback in the movie theater. On the contrary, DTS sound in the home cannot achieve the same level of sound quality achievable through a DTS moviehouse installation due to the relatively reduced bit rate of the former.
So where does this Dolby vs. DTS debate leads?
Sound quality is an extremely subjective issue and therefore it is not easy to define what constitutes better sound. Opinions vary considerably when comparing Dolby vs. DTS and range from ‘DTS is better‘ to ‘Dolby and DTS surround sound are pretty much the same‘.
…and what do we say about this Dolby vs. DTS debate?
Both Dolby Digital and DTS audio are capable of achieving similar end results in delivering surround sound, even though the lower compression/higher bit-rate of DTS Digital Surround should theoretically yields apparent benefits in sound quality. At the same time, one cannot ignore the fact that these two formats make use of different coding schemes and syntax to perceptually compress audio.
This means that efficiency in terms of data utilization between these two formats is different. Therefore, a Dolby vs. DTS direct comparison based solely on these formats raw bit rates cannot be taken as a measure of sound-quality.
Thus, while it is objectively possible to compare the resultant sound quality for the same audio format encoded at different bit rates, and therefore, to determine whether the same format in a moviehouse application sounds better or worse than in a consumer implementation in home entertainment, it is not so straightforward when dealing with different formats.
Rather, the reality is that for identically sourced audio content, it would be much easier for the listener during a Dolby vs. DTS ‘blind’ listening test to notice a change in sound quality when changing the playback equipment say between different brands, than when changing from a Dolby Digital to the DTS surround audio track.
In other words, you need a thoroughly refined home theater audio playback setup to bring out the subtle difference in sound quality between these two formats.