Secrets Q & A

TVs? We Don't Need no Stinking TVs - Third-Generation Multi-channel Audio - Part 3

ARTICLE INDEX

How a New Blu-ray Player Allows You to Keep Your Old AVR

Every six months new AVRs are announced by every major supplier. You may feel you need to upgrade but it is unlikely in an audio centric application. As mentioned above an HDMI 1.1 input is all you need. A correctly chosen Universal Blu-ray player has all the codecs and transcoders to deal with any disc or downloaded file. If you chose your new player properly, as described above, it should auto detect the formats and do all conversions automatically.

Ethernet interface to your computer music files can be done as easily with the Blu-ray player as the AVR. The same applies to listening to music files using USB.

THE IMPRACTICALITY OF WORKING WITH ANALOG OR DSD SIGNAL TRANSFER IN MULTICHANNEL

Audiophiles often ponder whether to use the analog outputs of a Universal Player or the digital transmission systems HDMI or S/PDIF (Sony/Philips Digital Interface Format). Electing for the DACs in the Blu-ray player requires coming in to the AVR with the six or eight analog cables. This had been the only way to connect an SACD player owing to copy protection worries with S/PDIF. For DVD-A, the high-resolution two-channel data (up to 24 bits at sampling rates up to 192kHz) could be sent down S/PDIF, but multichannel data is compressed as it would be for any DVD over S/PDIF . A few manufacturers worked around the situation with proprietary links for the company's product line and, in most cases, their availability was limited to the more expensive units. Blu-ray Audio discs only send signals at a 48kHz sampling rate down the S/PDIF link.

The HDMI link provides an alternative to analog in all cases. It not only satisfies the copy protection issues required for SACD and Blu-ray, but also allows for lossless transport of seven channels with a 24bit depth at a 96kHz sampling rate (five channels up to a 192kHz sampling rate). Going to the trouble of making an analog connection can still be justified because the DAC in the AVR may not be as good as the one in the Blu-ray player. Jitter in clock recovered by the HDMI receiver to drive the DAC in the AVR may also be problematic. Early HDMI receivers had more jitter than S/PDIF because both audio and video were being transmitted across the same link.

Current HDMI receivers have lowered jitter specs by improving the clock recovery circuits. To reduce jitter even further, an asynchronous sample-rate converter (ASRC) can be added to the digital signal chain. An ASRC replaces the recovered clock from the HDMI receiver with a clean clock generated internally in the AVR. With very powerful DSPs available to AVR designers today ASRCs to be implemented with software ( sometimes called a soft ASRC). The presence of an ASRC function operating inside the DSP may not even be mentioned in a spec sheet. Even a service manual will not provide the information. Not all ASRCs have the same performance. The next reference is a somewhat dated overview of the issues from one designer's perspective:

http://www.benchmarkmedia.com/discuss/feedback/newsletter/2010/05/1/jitter-reduction-using-pll-and-asrc-devices

Designers of new Universal Blu-ray players make the assumption that nearly all users deploy the HDMI connection and, accordingly, components involving analog (audio and video) are being downgraded to keep the price of the player down. This includes audio DACs. Given its extraordinary analog performance, the Oppo BDP–95 is the exception among the group of newly introduced Blu-ray Universal Players. There is no reason an AVR could not match this performance; however, achieving state of the art performance from the DACs to preamp out is likely to be at the end of a long list of other features more visible to consumers. I estimate an minimum price of $5,000 if such an AVR or Pre/Pro is ever brought to market.

You still want to come in analog?

A barrier to a pure analog hookup is modern AVRs is the lack of a multichannel analog input. Again the engineers assume everybody will be using HDMI. If you find an AVR with these inputs, it is likely to be a higher-end unit with DACs comparable in quality to the Universal Blu-ray player, and is more likely to have added electronics to reduce the jitter on the recovered HDMI clock. No AVR has 9 or 11 channel analog inputs to support Audyssey DSX or DTS Neo:X

If you have a Blu-ray player with exceptional analog performance it may be lost in the analog stages of the AVR. Typically the internal circuit components that guide the multichannel analog signal to digital volume control (bypassing the DSP and data converters) have noise and distortion specifications that are higher in value than the signals at the output of a unit like the Oppo BDP-95. The op-amps and analog switches the AVR designer chose typically are low cost components matching those that are used for selecting a two channel analog input. Again the AVR designer is working under the assumption that HDMI is the preferred solution and the money for analog electronics goes to places that are involved in the conversion of the audio data on the HDMI line to digital.

Picking an exotic multichannel analog line stage to deal with a unit like the Oppo BDP-95 is beyond the scope of this article.

Even if your AVR has multichannel analog inputs, barriers can persist. For instance, you will not be able to use delay networks that compensates for the different distances between the speakers and the listening chair in the AVR. This is available only when the signal is in the LPCM digital domain. Similarly, bass management which is needed to deploy some "small" speakers that require the bass spectrum to be directed to the large-sized front channels or a subwoofer may be available only in the LPCM domain.

The delay or base management system in the Blu-ray player is the workaround. But, unlike an AVR, the Blu-ray player has no auto calibration with a microphone. You must measure the distance to the listener's position, manually identify which speakers are deployed, and specify if they are large or small. In most Blu-ray players, the bass management system is simpler, with no opportunity to select the crossover frequency for different speaker groups. Only an 80Hz crossover will be typically available. Again the Oppo BDP 95 is an exception offering more crossover options.

Problems For Those Who Want to Work With DSD

Some audiophiles consider it important to keep the digital data from an SACD in the DSD format until it is converted to analog. One way to do this is to use the DAC in the Blu-ray player and come out analog despite all the problems cited above. Before you consider this route make sure the Universal Blu-ray you are using has a DAC that will accept a DSD signal as an input. Some products just transcode DSD to LPCM (Linear Pulse Coded Modulation) and send the LPCM to the DAC. All Blu-ray players I know of provide no delay compensation and bass management for DSD data signals. To access these you must allow the Universal Player to transcoded to LPCM first. This should be in the setup menu but some players may just do this automatically if you activate the bass management panel.

Note, not all AVRs accept a Direct Stream Digital encoded signal from an SACD over HDMI. An HDMI receiver that is 1,2 complaint is required to receive DSD (the different versions of HDMI are explained in the sidebar). If the AVR does not support HDMI 1.2 the Universal Player must first transcode the DSD signal to LPCM.

Even if AVRs does take DSD in you have no guarantee that it will be sent directly to a DAC that has a DSD input. Many AVRs will just transcode DSD to LPCM at the HDMI 1.2 receiver. Spec sheets and instruction manuals for AVRs that support DSD may not make this clear. Sometimes, I have resorted to block diagrams in service manuals for clarification.

There is no standardization for the DSP block that transcodes DSD to LPCM. It is easy to test a transcoder's performance by setting the Blu-Ray player to transcode DSD using an SACD test disc as a source. Suboptimal design may be signaled by degradation of high frequency bandwidth, SNR, or distortion at the transcoder output. Measuring transcoder performance from the HDMI output requires test equipment that will take HDMI in. This equipment is very expensive designed mostly for quality control at the production line. Measurements at the analog outputs does not give the whole story because digital-to-analog converters may have different performance characteristics when drive with DSD than they do driven by LPCM. The audioXpress review of the Oppo BDP-83 by Charles Hanson includes measurements of the transcoder at the analog outputs:

http://www.audioxpress.com/reviews/media/hansen3207.pdf

No problems were indentified with this unit at the analog outputs.

An AVR that accepts DSD direct provides only analog volume and balance adjustments with no distance compensation. Some AVRs that accept DSD may have a primitive bass management system with digital filters for the subwoofers activated in DSD mode (the band of frequencies to be sent to the subwoofer are analog-to-digital converted and sent to the DSP). A fixed 2nd order analog high-pass filter is placed in the main channels. Spec sheets and instruction manuals for AVRs supporting DSD may be vague on bass management when DSD conversion at the DAC is desired. In most cases only the left and right channels have the analog HPF available and only the left and right channels are converted to digital for the subwoofer bass management function. In other words in most cases the bass management for DSD only works for stereo. Again I have had to resort to block diagrams in service manuals for clarification.

Digital Room Correction Connection Issues

In the past, multichannel room correction in AVRs has been very expensive or very bad. Now, solidly-performing room correction systems are embedded in AVRs starting at $1000. A good room correction will clean up the room-dominated effects below 300Hz and select the optimum slope for the crossover between the main channels and the subwoofer. A standard bass management system has a 12dB octave High Pass Filter (HPF) and a 24dB octave Low Pass Filter (LPF). This filter set. almost always results in large frequency response changes in the crossover area.

Above shows the electrical response of an AVR with bass management selected. The red curve is the frequency response of the bass management filter applied to the satellite channel (12dB/oct). The Blue curve shows the response of the bass management filter applied to the subwoofer channel (24dB/oct). With a good room EQ turned on the frequency response curves (black and green) vary significantly from the standard bass management curves filling the dips and peaks in the crossover area. Done correctly this will yield a flat frequency response at your listening chair in the crossover region.

Some room EQ systems coming on the marker offer support of multiple subwoofer to allow for a flat subwoofer response over more listening seats.

Digital room correction above 300Hz allows matching of frequency response of all dissimilar-sized speakers. Most important is center-channel frequency response. Normalization by the room correction is required when using a dedicated center channel above or below the screen. Room correction cannot change the distortion to the image from a center channel not aligned in the vertical plane.

You must use an HDMI input if you want to use your AVRs room correction system. Room correction is done only in the digital domain. If you choose to go analog into your AVR or chose not to transcode DSD streams to LPCM room correction will be bypassed.

External multichannel room correction systems for consumers have analog inputs and outputs but the internal operation are done in the digital domain. A new pair of ADCs and DACs are introduced into the signal chain.

Note the only analog audio output you have from the AVR is preamp out. This is post the volume control which makes the conversion of the signal back to digital in the external process very challenging if dynamic range is not to be compromised. Since an AVR does not have inputs to the power amp once you go with an external room EQ you also must use an external power amp.

In the recording studio external processors are connected in the digital loop.

The difference is that no copy protection is used in the digital communication links for production. Multiple S/PDIF lines are the simplest way to move multichannel digital around the studio provided clock synchronization is maintained between lines. HDMI is never used in the studio.

In the absence of HDMI anywhere but the input of an AVR the only way to avoid extra ADCs and DACs in the signal path is for your room correction system to be internal to the AVR.

An older AVR may not support a signal sampled at a rate of 192kHz. Some can discern when a signal is down-sampled to 96kHz but not everybody. There will be silence when connecting at source with a sampling rate of 192kHz via S/PDIF to a unit that only processes a signal at a 96kHz sampling rate. Only HDMI down-converts as a result of bi-directional communication. If you are coming in S/PDIF from a computer the program you are using should offer to down sample to 96kHz.

Room correction is one area that may push you to upgrade. These systems have significantly improved on some new AVRs. Other new AVRs continue to be shipped with room correction that is better set to off.

If you are just starting out you and are willing to think about purchasing used you will find great deals on older AVRs especially if they only support HDMI 1.1. Older high end AVRs and Pre/Pros suffer especially badly in the used market. Something completely out of your price range new 3 years ago may be affordable now. Those who insist on exiting the Universal Player with an analog connection can save even more by looking at high-end equipment that lacks any HDMI inputs.