Dolby Pro Logic II vs. SRS Circle Surround

      转自： http://broadcastengineering.com/mag/broadcasting_dolby_pro_logic/

      In the same way that digital technology has replaced analog,surround sound is replacing stereo. Phenomenal sales of DVD-video players and home theater systems confirm that consumers enjoy sound from all around. Savvy broadcasters understand that they must some how squeeze 5.1 channels through their 2-channel audio signal paths.

      While matrix surround systems such as Dolby Pro Logic II (DPLII) and SRS Circle Surround (CS) cannot perform as well as discrete digital systems, they are effective solutions for many broadcasters. To learn more about the operation of each system, and in particular to test the sonic performance of them, we assembled a Dolby DP563 encoder and DP564 decoder, and SRS CSE-07 encoder and CSD-07 decoder, and put them through their paces.

      The well-known Dolby Pro Logic (DPLI) technology was first introduced in 1987, allowing broadcasters, via 2-channel audio delivery, to deliver surround sound audio to home users. Subsequently,Dolby introduced its improved matrix surround technology, Dolby ProLogic II. DPLII allows the encoding and decoding of stereo surround channels with full bandwidth, with improved channel separation and more intelligent logic steering mechanism. It also permits flexible mixing of the LFE channel so that broadcasters can tailor the signal to the end users' needs. As in DPLI, home audiences can switch between stereo and surround sound playback of matrix-encoded content, depending on their preference and playback systems.

      SRS burst onto the scene in the late 1990s with its CS technology, a conceptually similar matrix encoding/decoding system. The company has expanded its market share by securing deals with several major broadcasting networks. CS claims to offer a more versatile system that delivers multichannel audio from any stereo, matrix-encoded (includingDPLI/II) or CS-encoded content. It provides full bandwidth in all channels; the capability to encode an L, R, C, Ls, Rs, Cs or LFE signalas the dominant channel; dual-band steering logic for good channel separation and smooth steering between Ls and Rs; and front and surround channels. Moreover, its decoder is bundled with other proprietary SRS technologies such as Dialog Clarity and TruBass forcenter channel and bass enhancement.

       Both the Dolby and SRS systems are fully backward-compatible with mono, stereo and surround. (CS is also compatible with DPLI/II). This compatibility makes this matrix surround technology a powerful and flexible way to deliver realistic surround sound for broadcasting,film, music and game applications, as well as Internet streaming. In other words, both are very viable products.

      But because DPLI/II-equipped A/V receivers can be found in millions of household home theaters, can CS, the new kid on the block, compete with the more established technology? One key to potential success is sound quality. To find out if either has a sonic edge, we set up both systems, ran identical source material through both encoder/decoder pairs and listened to the results.

The hardware setup

The systems were set up in a post-production studio equipped withtwo Yamaha 02R consoles, ProTools, audio interface in/out and othertypical gear, as shown in Figure 1.

Figure 1. The listening test comparison ofthe Dolby Pro Logic II and SRS Circle Surround matrix audio systems wasconducted in an audio/video post-production studio. Click here tosee an enlarged diagram.

      Next, the SRS encoder was connected to a hard drive recorder audiointerface output; six analog channels were directed to the encoder viabalanced inputs. The encoder's outputs, (total left channel and totalright channel) were directly connected to the decoder's inputs with twoanalog cables. Six distinct outputs from the decoder were then directedback to the hard disk recorder so the encode/decode signal could bestored for playback.

       Setting up the SRS system was a simple task, which is a significantadvantage. The signal path was tested with the encoder's internal testsignal. The system was calibrated by adjusting the output levels ofeach channel using gain trim controls on the front panel of thedecoder. (There are no level adjustments on the encoder.) The system isdesigned to drop into an analog signal path; the encoder has analoginputs (its own A/D), and the decoder has analog outputs (its ownD/A).

        The Dolby system was connected in a similar manner. However, becausethe DP563 encoder has only digital BNC inputs and outputs, anadditional A/D converter was needed to convert the six analog channelsfrom the audio interface into three digital channels for input into theencoder. (We used a Dolby model 587 multichannel audio converter, butany 8-channel converter can be used.) Controls on the encoder's frontpanel provided access to setup parameters such as monitor status andspeaker configuration for system testing. Also, the decoder can beremotely controlled. Altogether, it is more sophisticated and flexiblein operation and configuration than the SRS system. However, extrafeatures and flexibility also increase complexity and require moresetup time.

     

The listening test

       To conduct an impartial A/B listening test comparing the Dolby andSRS matrix surround systems, we used our own source materials: twooriginal surround recordings and several commercially available DVDmovies. We also created special files for our very own matrix surround“torture” test.

        We began by listening to a passage from a live recording of Mahler'sSymphony No. 5. Both systems performed equally well in terms ofoverall pleasantness. However, there are some fairly audibledifferences between the two. The CS-decoded version accuratelyreproduced the original with a more diffused and spatially widersoundfield across the front speakers compared to that of the DPLIIversion. Different sections of the orchestra and choral could be easilylocalized, yet the sense of envelopment was preserved.

The DPLII version outperformed CS with a more accurate reproductionof the original spectrum; the balance of the high-frequency range tothe low- and mid-range was well-maintained. A bass boost was observedin the CS version, which imparted a fuller sound but also at times“muddied up” the mix.

        However, the most significant difference between the two systemsoccurred at the end of the recording. The audience applause, which wasmixed predominately to the rear surround channels in the originalrecording, was distributed over the front channels in the DPLIIversion, while the CS version preserved the front-rear channel balancemore accurately compared to the original with just slight leakage tothe front speakers.

        Turning to rock ‘n’ roll, we chose an original surroundrecording that had very little center channel content, as is the casein some surround recordings. We immediately identified the bass boostin the CS-decoded version. Because rock music typically has greaterbass content compared to classical music, the boost was clearlynoticeable and sometimes overpowering.

Both systems handled the “phantom center” quite well; nosignificant spatial artifacts were observed. The CS version had moreseparation across the front speakers with good clarity that was similarto the classical example, while the DPLII version had more emphasis inthe middle, which provided a better sense of definition andpresence.

         We next auditioned several sound-tracks (from DVD) filled withexplosive sound effects. The DPLII version excelled with accuratereproduction of the originals in terms of transparency and dynamicsspectrally, and surround envelopment and localization spatially. It hada tight and punchy bass that was more than sufficient for rumblingexplosive sound, while the high end was crispy and sharp. Spatially, itprovided excellent separation between all speakers with excellentlocalization of panned sound sources in all directions.

The CS version, on the other hand, had a ground-shaking low end forthe explosions but a slightly darker high end compared to that of theDPLII version. It had a much diffused soundfield and smoother panning,which worked quite well for ambient sound. However, it suffered frompoor localization due to interchannel cross-talk. For example, circlinghelicopters sounded like they were coming from all directions.

        Finally, our torture test. To stress the steering logic of the twosystems, we sequenced and recorded a series of kick drum hits bouncingbetween the front channels (L-C-R), the rear channels (Ls-Rs), frontand rear, and between the LFE and the five main channels. TheDPLII-encoded version excelled in this category. Interchannel crosstalkwas minimal with few spatial artifacts, especially between the fivemain channels and the LFE channel. With only a slight clipping in thecenter channel and double flaming of the drum hits (due to a default15-ms rear surround delay), the DPLII system passed our testeasily.

       The CS steering logic, however, seemed to be confused by the test.With an apparently slower reaction time, the CS-coded version exhibitedsevere crosstalk between all speakers and abruptly switched contentfrom the LFE to the main channels. In this torture test, weparticularly preferred DPLII.

Summary

       DPLII and CS are competing products, but each offers advantages and disadvantages to a potential buyer. Because of the built-in A/Dconverters in the SRS CSE-07 encoder, it and the CSD-07 decoder are easy to drop into an analog signal path. Moreover, it is fast to set upand simple to operate.

       Because of its digital-only input, the Dolby DP563 encoder is more suited to a digital signal path. It and the DP564 decoder, at the expense of complexity, offer greater operational sophistication.Finally, at least in our listening tests, although neither system is as transparent as discrete digital coding, we generally preferred DPLII over CS. Both systems provide surround sound over any 2-channel serviceand accomplish a significant upgrade from stereo to multichannelsound.