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Is there much of a difference between using optical vs. digital coax for transmitting legacy Dolby Digital and DTS sound from a Blu Ray player to an older non-hdmi receiver? I heard from some people that digital coax can handle much more bandwidth than optical, therefore sound will be better. Some have also said that digital coax can carry Dolby TrueHD and DTS Master HD signals, but I'm not sure if I believe that, since I imagine bandwidth of those new formats is proably 100 times more. I also heard that Blu Ray disks nowadays have a higher quality legacy Dolby Digital and DTS track than DVD's do. I'm not talking about Dolby TrueHD or DTS Master HD, which are lossless, but plain old DD and DTS. Speaking of lossless, funny thing is the other day I was looking at some packages of digital coax and optical cables in a store and it said "lossless sound" on the package. How can this be? I thought only Dolby TrueHD and DTS Master HD are lossless. Sounds to me like marketing gimmick.

Have you ever heard of Optical to Digital Coax converters, or the other way around?   I thought that a straight line connection between say a Blu Ray player to receiver is best, but what if you have a converter in the middle, like that which I mentioned above, or even an HDMI converter?  Is sound degraded at all?

Thanks for all your help.

Answer
Optical (also called TOSlink) versus coaxial, should be identical from a functional perspective - the same data is being sent on either format. However TOSlink offers an advantage over coaxial in that it does not make an electrical connection between the two devices, which can eliminate ground loops (it carries no conductive connection between the two devices, as it is just a plastic or glass fiber; some cables may have gold flash on the tips, but this does not continue through the cable and is generally done for cosmetic reasons by the cable maker) and many interference issues (it is immune to RF/EM interference - although keep in mind that a proper specification coax cable is shielded). In general TOSlink is "better" as a result, but there should not be any noticeable differences unless it eliminates a ground loop in your system.

Regarding the rest:

- Coaxial and TOSlink as S/PDIF are both limited to 1536kbit/s. There is no bandwidth advantage to coaxial or TOSlink. There should be no sonic differences between the two either (they're carrying the same signal) - if there are, I would look at the specific equipment producing the differences, as it is likely either applying some form of filtering/processing to one form and not the other, or is defective in some way.

- Even if coaxial carried higher bandwidth, or for more a realistic example, sending Dolby Digital over HDMI, there would be no performance advantage - you will still be getting the same signal, and it doesn't matter if the signal travels over a connection that fulfills its bandwidth requirements 1:1, or if it has excess bandwidth to spare. The data being sent isn't changed.

- Coaxial cannot carry Dolby TrueHD or DTS-HD Master Audio, nor can it carry multi-channel PCM, or any other lossless multi-channel signal. HDMI is the only connection that can handle this.


- The bitrate differences between Dolby Digital (AC-3) and DTS from Blu-ray or HD-DVD and DVD are relatively small. AC-3 from DVD has a specified maximum bitrate of 448k, while HD-DVD provides for up to 504k, and Blu-ray up to the AC-3 maximum of 640k. DTS on DVD is generally encoded at 754k, however can be encoded at 1509k (this is relatively rare). This will translate equally to the core extraction from Blu-ray or HD-DVD titles with DTS-HD tracks.

- The HD formats can be encoded at up to around 20 Mbit/s, however only TrueHD and DTS-HD Master Audio support this functionality (Dolby Digital Plus and DTS-HD High Resolution are separate; they are relatively rare on Blu-ray, but Dolby Digital Plus is relatively popular for HD-DVD - they also are generally set at around 3 Mbit/s).


- Blu-ray and HD-DVD titles can carry conventional DTS and AC-3 audio tracks (and many do; Tomb Raider on Blu-ray and Sky Captain and The World of Tomorrow on HD-DVD are examples of this), however if they do not they are required to support backwards compatibility. DTS-HD itself carries a "conventional" DTS audio signal within its data-stream (due to how the DTS encoder works, all DTS modes are extensions built upon the base 5.1 codec), while Dolby TrueHD discs will carry an additional Dolby Digital track for legacy support. The quality of these tracks is equivalent or slightly beyond the bitrate found on a DVD. Discs that forego carrying a lossless encoded track many alternately carry a multi-channel PCM track (which is lossless, and generally VERY high bitrate, although it is supported by a wider range of HDMI equipment) and will generally also provide a DTS or Dolby Digital track for compatibility (Pirates of the Carribean on Blu-ray is an example of this).

- The lossless claim on coax and TOSlink cabling is perhaps misleading, but is not wholly inaccurate. S/PDIF was originally developed to carry digital audio from CD players. This is a lossless audio signal, albeit a stereo one. In this mode, the audio being transmitted is 44.1khz 16-bit stereo PCM at 1411 kbit/s.

- Lossless audio itself is not the sole domain of Dolby TrueHD or DTS-HD Master Audio; it simply refers to a digital encoding technique whereby the original input signal can through the encoder without loss (hence, lossless) - the waveform that comes out upon the signal being decoded should be mathematically identical to what was originally input. Lossy encoders use a variety of techniques to produce a smaller file size from the same material, and generally this is done in a transparent manner (meaning that humans cannot detect a difference); generally you will only hear compression artefacts with overly-compressed material. For example reducing a CD to 96kbit/s will often result in some artefacts. In general, Dolby Digital and DTS are considered to be transparent at their conventional bitrates, and lossless codecs provide little (if any) performance advantage by comparison.

- Optical to coax converters (and vice versa) are a very real thing, and exist for compatibility reasons. Not all devices that sport digital outputs sport both TOSlink and coaxial, and not all devices that accept digital inputs sport both as well. Converting between the two is not that big of a deal in concept; any TOSlink receiver will perform the same conversion on its input stage (the signal has to come down to copper eventually) - such converters are generally just a combination of a receiver and transmitter, they're fairly simple devices and generally don't cost very much. There isn't any need for one unless you need to convert signals for compatibility reasons.

- There should be zero perceptible difference between converting the signal and connecting it straight through; digital is generally an all-or-nothing affair. Either the signal goes through (and that's the best you will ever get) or there is no signal. There isn't the gradient of performance that you see with analog transmission. The error rate essentially can go from 0 to whatever the threshold is with no impact, and then the signal drops out entirely. That said, some devices may deliberately alter the signal (usually the device will mention this as a selling point), which should result in a perceptible difference.


In general lossless vs lossy audio translates to very minor, if perceptible, differences. The primary advantage to newer codecs are support for more discrete channels of audio (Dolby Digital and DTS only support up to 6 discrete channels (5.1), and 7 channels via matrix (6.1); current HD codecs support at least 8 channels (7.1), and include up to 14 channels of support via future expansion). There is no disadvantage to an HD codec, the benefits are generally very minor relative to the upgrade cost however, which should be weighed if upgrading equipment. In general if the only reason for upgrading equipment would be to support HD codecs, I would suggest passing on the upgrade, however there may be other benefits to newer equipment such as improvements in power efficiency, video processing, automatic room correction and calibration, network connectivity, and multi-room/multi-zone integration features.

If you have further questions or need clarification, feel free to ask.

-bob

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Bobbert

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Questions regarding HTPC integration to home theaters, and general purchasing advice regarding home theater and audio systems, including headphones. Please no car audio or over the top PA systems.

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General enthusiast, ~10 years as an audio and electronics hobbyist

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Engineering student, various DIY experiences, personal hobby

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