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Bellevue, WA 98007
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Video for Audiophiles
By, Amir MajidimehrThis is a presentation I made to the Pacific Northwest Audiophile Society. As the name implies, they are heavily into audio. So I thought it would be good to present video in the context of what audiophiles would want to learn. The presentation is useful to all but if you know audio well, it will especially resonate.
As you see in the slides, every topic from how we digitize the video, to its data rate, encoding, compression (MPEG-2, MPEG-4 AVC, VC-1), transmission (HDMI), projection technology (LCD, DILA, DLP), calibration and most importantly, high fidelity audio for video is covered.
The original presentation had 3-D animation videos for the room simulations. This version does not. I will upload the rendering together with a dedicated article at a future time.
Audio and video are quite different in end-to-end production flow and achieved fidelity and usage in the home. For example, unlike audio where we have no idea of "truth," we can with 100% confidence determine whether what we see at home is what was produced in mastering. This is accomplished by using strict standards that simply do not exist in audio. On the other side of the coin, video achieves far lower performance than audio which fortuitously matches our poor eyesight relative to our hearing perception.
Digital video is created by separating black and white and color components (Luma and Chroma respectively). This is done because the eye is much less sensitive to resolution in color versus black and white. As you see from the computation below, the number of bits that represent our video is quite low relative to audio. Specifically the black and white samples have only 8 to 10 bits as compared to audio’s 16 to 24 bits. The resulting signal to noise ratio is a poultry ~48 dB which is far cry of 96 dB we get for CD audio for example.
As you see in this slide, the total number of pixels (dots) in our video, even in high definition, is extremely low relative to our typical still image capture devices. At just 2 million pixels, HD video is a far cry from even the cheapest digital cameras. Yet, we enlarge the video to such larger frame. Imagine trying to print a 10 foot wide picture that is just 2 megapixels! Video takes so much data that capturing and delivering video at higher rate is quite challenging so we are stuck at this limit for some time. Fortunately, as can be seen in our reference theater with its 17 foot wide screen, the image can still be breathtaking if done right.
Using the details already provided, we can compute how much data we need to store to represent digital video. As you can see, without some kind of compression the numbers are insanely high. At 389 Gigabyte, a typical laptop hard disk can’t even store a single movie!
The basic concept of video compression is easy: compress all we can in a still video frame and then transmit what changes from that frame to the next. For example, if there is a solid white wall, we can reduce the amount of data it takes to represent all of those similar pixels. But importantly, if the wall stays the same from frame to frame, we can simply tell the decoder to repeat them and thereby, save a ton of data in not having to retransmit that redundant data over and over again.
The image on the left is the original. Compare that to the heavily compressed version to the right. Notice how the distortion is highest on the edges which we call “high frequency” portion. The original image is 3 megabytes and the compressed, just 0.03 megabytes or about 100 times smaller.
Close up of the compressed image. Note the "blocking" artifacts on the face and extreme distortion on all the edges.
We have come a long way since the original video compression standard, MPEG-2. We can achieve double the efficiency enabling us to deliver better quality in less space and with reduction in bandwidth required. Compression standards such as MPEG-4 AVC and VC-1 (also called WMV) enable Internet delivery and Blu-ray Disc to perform much better than if they had remained MPEG-2. Unfortunately, MPEG-2 remains the standard for US television digital standard and hence the horrendous artifacts present in it especially in sports where the high motion and detail become distorted. Standards are great in lower the cost of products but they also stifle innovation this way as we will have to live with this transmission system for long, long time.
As mentioned earlier, in video we have an end to end standard that allows us to verify every piece of the deliver chain to make sure it stays faithful to the original content. This is done by using a special color pattern (pictured) below at original capture point. If we make sure that the captured video is producing the preset values in that chart, and using measurement equipment at the display, we can assure that what we see is what was captured.
As mentioned our job at high level is simple in achieving full fidelity: we feed the display the color bars and measure whether each color component is where it needs to be. You can see that in the CIE chart on the left where the dot is the measured value, and the square what it should be. If they land on top of each other, we are golden. If they do not, then the display needs to be adjusted. The example below is from special software we use to simplify this work (Calman). The input to the system is a sensitive color spectrometer. There is more to this of course than this brief introduction including such concepts as “gamma” which determines how the display shows different gradations of brightness.
The concept of 3-D video is really simple: we simply need to capture two video streams, one representing what the human eyes would have seen. The brain then uses the same technique it normally uses to determine depth. But at home, we only have one display. To simulate two, we (usually) play video at twice the rate, with frames for each eye alternating with the other. Then by the use of active glasses, we can make sure at any one instance, only one eye can see the video intended for it. Passive systems used in theaters use two projectors with the light polarized 90 degrees from each other. Combine that with a set of polarized glasses with the same design and you make sure that each eye only sees one image again. The drawback here is that you need a specialized projection screen that preserves this polarization. These screens do not work well for 2-D viewing which is most of what we watch. Expensive solutions exist such as using two screens but for the most part, it is best to use polarized glasses and a single projector such as our Sim2 3-D Nero and Solo.
The best way to experience movies at home is with a large screen and that is enabled using a projector. For example, the screen we have in our showroom is a whopping 235 inches diagonally (how displays are usually measured). Compare that to your 5 or 60 inch flat panel. By completely covering your field of view, we achieve what is called “suspension of disbelief” making you feel like you are in the movie, rather than sitting at home. Combine that with surround sound and the illusion is complete. There are three competing projection technologies LCD, DILA and DLP, each with their own pros and cons as listed in the slide. Key metrics are image sharpness, contrast, black level (how dark the image can get), 3-D performance, etc.
Projector can either use a single imaging element to synthesize the image or three. In case of the latter, each represents one of the primary colors (red, green and blue). While this has some advantages in the purity of the color and contrast, the drawback is that the three panels need to align perfectly or the colors bleed. In the picture below, you see how far off each color can be from each other, rather than at the same spot. This is on a $60,000 projector! Thankfully there are much higher quality projectors such as our Sim2 which have near perfect “registration” (alignment of pixels). All LCD, DILA and some DLP projectors use triple imager technology.
DLP projectors can be designed with a single image where a color wheel in front of it in sync with each color can convert the image from black and white to color. Advantage here is that there is no misalignment of panels so the image can look exceptionally sharp. The other major advantage is reduced cost. To wit, our Sim2 Nero 3-D projector is half the price of its 3-chip 3-D Solo! The only drawback can be for a small percentage of the population which can sometimes see a strobing effect where the colors separate for a moment. Thankfully by spinning the color wheel very fast, this can be almost eliminated as is the case with the Sim2 Nero.
The standard interconnect between display and monitor is HDMI. It is a twisted pair system designed originally for very short distances. That is a sever limitation when it comes to sending video to projector and to other rooms in the house. HDMI provides for automatic detection of remote devices and their capabilities although the latter is sometimes poorly done.
In many ways, audio is an afterthought in HDMI, being slaved to video. This causes extraction of audio clock to be difficult, often underperforming other digital audio standards such as S/PDIF by a factor of 10 or more! So for best performance, you may want to connect both HDMI and S/PDIF to your Blu-ray or DVD player.
HDMI reliability can be very poor. Problem is often blamed on the cable but usually is the fault of improper implementation at the source, processor/AVR or the display. Troubleshooting requires having proper equipment which sadly almost none of the design and installation companies, sans us, own. Without it, it is an expensive and time consuming process of swapping out equipment until the faulty unit is identified. Often the problem can occur down the line where a fully functional unit stops working when say, a component like a DVD player is replaced and all of a sudden the video flashes, becomes green, or no picture is shown.
While audio for video shares the same goal of high fidelity reproduction as does 2-channel music, it differs in many respects. Theaters are often bare, making them less acoustically suitable for good sound reproduction. Most importantly, movie sound is bass heavy making reproduction of low frequencies much more important.
The typical approach is to just throw some speakers in the room and sometimes, following incorrect advice on the Internet on how to treat the room. These are all random approaches to a problem that can be solved scientifically to be correct at the start, and to produce the flattest and most faithful reproduction of bass frequencies.
Textbook approaches to room acoustics abound. Unfortunately they are for the most part wrong because they make assumptions such as ideal rectangular rooms with completely symmetrical wall construction and no furniture which does not often occur in real life.
The right approach uses two key techniques: use of multiple subwoofers and fluid dynamics simulation of the room. Multiple subs is critical to get around the fundamental physics of waves reflecting from walls and adding/subtracting and with it, create wildly varying frequency response which varies from seat to seat. Using more than one subwoofer as later slides show, can sharply reduce these variations but this requires knowing where to place them and that is where the second method comes in. Using computer simulation of speakers as pistons energizing the room and the air as the “fluid,” we can model thousands or even millions of configurations of subwoofer locations, arriving at the best location and number of units to get the best response.
The proof is in the pudding as they say. Here are the simulations of our theater with a single subwoofer on the left model, and three on the right (two on each side and one in the ceiling). I will post the full video later but for now, you can see in this frozen frame at 17 Hz, how different the room response is between the two. The colors are the “isosurfaces” of similar pressure (equal loudness).
While the isosurface simulation looks pretty, perhaps a more useful view is the cutaway that shows the pressure at the ear height only. After all, it is not important how the room sounds at other points in the room. Here we see a more simplified but more dramatic view of what is occurring in each subwoofer configuration. In the single subwoofer simulation on the left, at this 30 Hz frequency point, there is a whopping 25 dB of difference in loudness compared to just two seats over! Imaging you hearing normal bass while your wife going deaf sitting next to you hearing this much exaggerated frequency response. In contrast the three sub simulation shows almost zero difference in sound pressure at this frequency between the seats. This is the ideal that we want to strive for and there is no way to achieve it with one subwoofer and without simulation. Yet, the most common system sold for home theater is a single subwoofer given to the homeowner or the installer to randomly place in the room.
Whether you want us to design your theater or yourself, we can provide this turnkey service for you. Using simple measurements of the room and all the allowable positions and maximum number of subwoofers, we can then run the simulations for you and provide you with the optimal number and placement of subwoofers. For the cost of one or two subwoofers, you will have, far, far better sound than any other solution even if you do nothing more than this.
There are two programs: Gold and Platinum. The differences are listed in the slides. The cost difference is 2X.
Once you have the subwoofers optimized the next step is to use acoustic treatment to deal with remaining variations which should be much smaller.
Final step is electronic equalization. There is no more powerful system than the JBL Synthesis system using the SDEC processor. This is a 20+ channel audio system that can concurrently correct that many audio channels, allowing it to easily accommodate multiple subwoofers and bi-amplification of speakers. The SFM system here will attempt to deal with variations between subwoofer size and model, and ARCOS provides optimizations of speakers and subwoofers.
Here you see an example of SFM/ARCOS smoothing the response of the subwoofer (the thicker solid line) relative to the more distorted version prior to correct (the more faded lines). As you see, the new response is much smoother. Note how JBL shows you before and after whereas consumer room EQ systems which never provide you measurements post correction. They only show you a pretend graph of what they told the speakers to reproduce. Such graphs are notoriously wrong in those systems and hence the reason they usually do more harm than good.
I wish our audio/video systems worked perfectly out of box but unfortunately they do not. Worse yet, figuring out that they do not and how to get them to perform is an expensive proposition. As an example, to measure display fidelity we use our Minolta spectrometer which originally cost us $20,000. No wonder then that companies don’t bother measuring systems they sell or use cheap consumer devices that simply do not have usable precision as the display brightness goes down (critical to make sure the display does not have a color shift in darker areas).
Likewise, as mentioned earlier, troubleshooting HDMI requires having an instrument such as our Quantum Data HDMI portable analyzer. We have solved customer problems in 30 minutes using this instrument that we could not with weeks of swapping equipment and blaming the wrong gear for the problem.
Neither system is easy to use however. They are professional tools requiring extensive knowledge of what they do and how the underlying system operates. Thankfully we are here to get the job done so the point is to make sure whoever you use for your audio/video needs is similarly situated. Or else, you are headed for reliability and performance issues.
This was the lead in introduction to our reference theater employing the techniques mentioned earlier for its design. You can read more about it in our article on the design of our reference home theater.
And here is a picture of the front wall,
showing the speakers, one of the subwoofers and special acoustic
treatment in the front wall and on the sides.
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