| Originally Posted by Mark J. Foster This e-mail is really targeted at the designers of the AVR-4806. In a nutshell, when decoding a 2-channel Dolby Digital signal, the AVR-4806 does not properly complete the Dolby Digital decoding stage, leading to a lack of stereo separation in the decoded signal. To help you identify the problem area, I've prepared a special pair of audio files that make it very easy to see what's wrong. These files contain a special data format that appears as a perfect circle when viewing the Left Front and Right Front channels on an X-Y oscilloscope. To illustrate the problem as clearly as I can, I'm also attaching oscilloscope screen shots that show the problem. Specifically, here's a view of the EFQPSK.WAV file when plotted with digital loopback from a sound card, that is, playing in Windows Media Player, with essentially perfect reproduction: http://www.talix.com/RX/EFQXY-SBXDSD.PNG Next, if this signal is sent to an AVR-4806 via one of the Line-In inputs, and we plot the Line-Out outputs of the receiver, we'll see how the signal looks after it's been AC-coupled (with variable decay turned on to let you see how the signal changes over time). The signal exhibits some "dispersion" simply because of the AC coupling (in other words, the circle moves around over time, as the random data is predominantly high or predominantly low): http://www.talix.com/RX/EFQXY-SBXDAA.PNG However, if the AVR-4806 receives the exact same 2-channel audio file, but the file is encoded as a Dolby Digital signal beforehand, the decoding process does not work properly! In this case, we're sending a Dolby signal via S/PDIF (optical or coaxial) to the AVR-4806, but watch what happens (in this case, shown with infinite persistence on the screen, just to make the result clearer): http://www.talix.com/RX/EFQXY-MYSDSD.PNG Since this is plotted in X-Y mode, as are all the shots, the angled ellipse shows a reduction in stereo separation, which is exactly the symptom of an uncompleted Dolby Digital decode. More important, I've found a huge hint that helps to explain exactly what is happening. I also own an AVR-5700. It properly decodes this signal into a circle, if it is in Dolby Digital mode. However, if the AVR-5700 is switched into Direct mode, it shows the same elliptical pattern that the AVR-4806 always shows. In other words, the AVR-4806, when detecting a 2-channel signal, incorrectly turns off the Dolby post-processing that is essential for proper reproduction! Of course, this testing must be performed with all "features" turned off, including THX processing, room equalization, and speaker distances must be set to 0' to prevent signal distortion by the various correction modes - only "Standard" Dolby Digital should be enabled. I'm attaching both the raw WAV file, as well as the same information encoded into a Dolby Digital .AC3 file, so that you may easily replicate these results yourself (both are contained in the attached EFQPSK.ZIP file). To playback the WAV file, just play it in Windows Media Player on just about any PC. To play the .AC3 file, you'll need an appropriate program that can perform S/PDIF passthrough of a Dolby Digital file. If you don't have an appropriate filter installed on your machine, I'd recommend the excellent, and free, open source AC3FILTER, which is available from SourceForge at: http://ac3filter.sourceforge.net After you've installed AC3FILTER, it will then be possible to easily playback the EFQPSK.AC3 file. Just make sure that you configure AC3FILTER to use S/PDIF passthrough! By the way, this is not the only Dolby Digital source that highlights the problem in the AVR-4806. If you playback the EFQPSK.WAV file on a sound card with a hardware Dolby Digital Live encoder, such as the Bluegears X-Mystique, or on an nVidia motherboard that contains on-board "SoundStorm" real-time Dolby Digital encoding, you will see the exact same problem. Other receivers that I have tried, including the AVR-5700, don't have this problem when they are set to Dolby Digital mode. I'm hoping that this information will be sufficient for you to isolate this problem, for this special WAV file makes it very obvious to see what's wrong! However, if you have any further questions or comments, you may contact me at: Mark J. Foster Thank you very much for your consideration of this important bug! |
