Well, you don't want to drown out your nice sound do you?
There are several different approaches to keeping your system quiet and cool, so we'll look at them all in depth.
Isolate noisy machine(s)
One of the easiest ways to keep your Myth system quiet is to (basically) throw it in the cupboard — don't worry, this isn't as drastic as it sounds. As your probably know, MythTV (like most bits and bobs of Linux) is highly network aware, and will allow you to run a separate backend and frontend. All of the noisy hot stuff, such as TV cards, hard drives, and Terahertz Athlons can be thrown into the backend and kept under the stairs; you can install a low-powered small machine under the TV. All of the control can be done from the frontend, which will just request TV streams from the backend over the network. The downside of this of course is that you need to be able to afford two separate machines, although it will make expandability much easier further down the road.
Viewing high-definition recordings requires much more processing power; decoding a 1920x1080 MPEG2 is much more intensive than viewing a 400x480 SD video of any format. It is thus more difficult to quiet a high-def frontend when you likely need a CPU over 2 GHz and/or a hardware video accelerator (IDCT via XvMC).
There is however an alternative for the cupboard approach. Several CAT5 KVM extenders exist on the market. Such a device extends your VGA, mouse, and keyboard cables using CAT5 network cable. You can extend up to 30 meters. One setup in use includes two Aten CE220 KVM extenders to extend my Dual Head development box from the hallway closet into the living room. This setup now has only two LCD screens, a mouse, and a keyboard sitting in the living room generating zero noise. It also includes an SPDIF cable running from the living room amplifier and a cinch TV-out cable to the TV-set.
If you're using an Athlon XP processor and your kernel is ACPI-enabled, you can use the Athcool utility (packaged for Debian), which works on all current chipsets, and will often drastically reduce your CPU's operating temperature. Unfortunate side effects can include crackly audio and slow hard disk transfers — YMMV. Some people say that these side effects have been eradicated or highly reduced under the 2.6 series of kernels — I suggest you try it out and see for yourself! A more verbose guide on Athlon powersaving mode is here
Even if you can't avoid having your quad 30GHz Xeon Myth array sitting under the TV, there are still many ways you can reduce the amount of noise today's computers like to kick out. There are many specialist computing companies out there specializing in quiet hardware for use in HTPC environments, and I'll list a few of the products you might want to splash out on:
Fan-mates, or variable resistors to you and me, alter the voltage supplied to your fans. Normal fans runs at 12V, but by lowering the voltage to 7V you will get lower RPM's and thus quieter fans. Air throughput is reduced however.
Fan adapters allow you to replace a high-RPM 60mm fan with an 80mm fan, which can be run at a lower voltage and still shift the same amount of air. By nature of their construction, these are best suited to replacing the 60mm fans on top of stock CPU heatsinks.
Get some bigger fans! This will often involve drilling some holes in your shiny computer case, and so isn't for the faint of heart. The basic principle is if you want to shift X amount of air in Y seconds, a bigger fan can do it at a lower RPM and thus will run quieter. Lots of modern HTPC cases opt for a few large fans rather than an array of smaller fans.
Quiet and/or adaptive power supplies can also help reduce noise by a considerable degree. There are some (highly expensive) power supplies that don't use any fans at all, and just use a huge heatsink for cooling themselves, and thus run totally silent. However, they don't do much for the cooling of your case! A better solution I believe are power supplies like the Tru Power range made by Antec. These dual-fan PSU's contain temperature sensors which raise or lower the RPM of the fans depending on how hot your system is. Many also provide "fan only" molex connectors to attach your case fans to; these are also linked to the thermal sensor, and will run your case fans at a reduced voltage unless your system gets too hot.
Stealth fans are fans especially designed to run quietly, and usually come at a considerable price premium over normal fans. Brand names include SilenX and Pabst (most fan manufacturers also have their own quiet fans sub-brand as well), and will usually run at much less than 20dB on standard 12V. Many of these fans have magnetic levitation bearings instead of roller bearings, and the lack of physical contact reduces noise *quite* a bit — you'd be amazed how much noise is transmitted by physical conduction.
Passive Northbridge heatsinks allow you to replace those pesky little 40mm fans on your motherboards northbridge with a solid chunk of aluminium. Be warned though that these can cause older northbridges to overheat if there is inadequate airflow in your case! Those of you lucky enough to be using Athlon64 or Opteron systems need not worry about this, because all of the hot bits that used to be in the northbridge have now been moved to the processor. Zalman has a range of northbridge heatsinks, but check they can be fitted to your motherboard first. Some motherboards, such as the ASUS A8N-SLI are designed to use heat pipes to remove heat from the northbridge and dissipate it through larger heatsinks that can be located in a less confined region of the motherboard.
Using a quiet heatsink on your CPU will also save your eardrums. This can be as simple as replacing the 60mm fan on a stock heatsink with an 80mm fan by way of a fan adapter as detailed above, or you can splurge on one of the excellent, quiet heatsinks from manufacturers such as Zalman. A cool Zalman solution is here. There are also fanless options available for completely eliminating the need for a separate fan on your CPU. For instance, the HR-01 from Thermalright.
Use a cooler processor! Several years back, CPUs kicked out a helluva lot of heat and caused systems to become uncomfortably hot. More recently, AMD and Intel have realized the error of their ways, and modern processors are faster than ever, but without generating enough heat to melt your case. Assuming you're using a hardware capture device (either a DVB card or one of the TV cards supported by ivtv) and a reasonably good graphics card, one of the good things about Myth is that 90% of the common functions don't require a powerful CPU at all, so you can often pick and choose the CPU that runs at the lowest power and heat output.
If you are building a new system, the least expensive processor you can find will be able to handle standard-definition video without any problem at all. Even cheap processors can handle high-definition video with only XVideo support from the video processor, particularly if they are dual-core CPUs.
If you are "recycling" an older system for a MythTV unit, your needed processor power depends on if you want to play back SDTV, 720p HDTV, or 1080 HDTV as well as your choice of video codec, whether your graphics card supports XvMC, and what deinterlacing you want to use. As a general rule, non-deinterlaced playback of 720x480 MPEG-2 files to a CRT TV using XVideo requires a 700 MHz Pentium III or 800 MHz Coppermine Celeron and playback using XVideo requires a 450 MHz PIII or Celeron. High-quality deinterlacing (such as GreedyHighmotion or Yadif) of 720x480 MPEG-2 files to play them on an LCD requires about a 2 GHz Pentium 4 or 1.5 GHz Athlon XP using XVideo output.
The only areas of MythTV that benefit from having powerful CPUs are MythMusic visualizations, CD ripping, transcoding, DVD ripping, and high-definition playback. If you're using software capture from a bttv card, you may also want a powerful processor.
Low power computing
Low power computing is running your processor at less than its full clock speed and voltage to produce less heat and use less energy. The heat produced by a particular processor increases linearly with increases in clock speed and quadratically with an increase in voltage (the change in heat production is the square of the change in voltage.) There are two ways to reduce your processor's voltage and clock speed. You can either have it done automatically if your processor supports such a function and you can do it manually.
Automatic CPU frequency and voltage scaling
Notebook processors have had the capability to reduce their clock speed and voltage while idle to prolong battery life for about 10 years. Recently this capability has been added to desktop processors as well. AMD calls their frequency and voltage scaling "Cool 'n Quiet" and Intel calls their "Enhanced Intel SpeedStep." Cool 'n Quiet allows the processor to drop to about 1200 MHz for the Phenoms and and 800 or 1000 MHz on the other processors. Intel Pentium 4 or Pentium D processors with EIST idle at 2.80 GHz while the Core 2/Pentium Dual Core/Celeron 4xx or E12xx processors idle at 1.5x their effective frontside bus speed, which usually yields idle speeds of 1.2, 1.6, or 2.0 GHz. Processors that support frequency and voltage scaling:
All Athlon 64 models
All Athlon 64 X2 models
Sempron 3000+ and faster on Socket 754
All Semprons 3200+ and faster on socket AM2
All Athlon LE processors
All Athlon X2 processors
All Phenom processors
Socket 940 revision E (90 nm) Opterons
All Socket 939, AM2, AM2+, and F Opterons
Pentium 4 6xx series except the P4 620 and a few early B1 stepping 6x1 series processors
All Pentium Ds except the 820 and 915/920
All Pentium Dual Cores
All Core 2 Duos, Core 2 Quads, and Xeon 3000 series
Celeron E12xx series
Activating the automatic frequency and voltage scaling is accomplished by loading the appropriate cpufreq drivers for your chip and setting the cpufreq governor to "ondemand." This should be automatically done for you in most Linux distributions made in the last few years.
Manual CPU frequency and voltage scaling
Those of you who know about overclocking may well want to experiment with underclocking and undervolting. This involves manually adjusting the CPU voltages and frequencies to a level below the stock voltages and frequencies. That can be accomplished by going into the BIOS and setting these values if your motherboard allows it. Some motherboards do not allow any frequency or voltage modification while others will only allow for you to overvolt the processor (such as abit boards.) You can also reduce the processor's operating voltages adjusting the EIST/Cool 'n Quiet voltage values using a tool like cpupower or Linux-PHC. Note that these tools are generally only useful for adjusting the voltages for the above-idle speeds, particularly on AMD processors as the voltage for the idle speed is typically not adjustable.
Buying a specially-designed energy-efficient CPU
Intel and AMD both have lines of CPUs specifically designed to run at a lower thermal dissipation than standard CPUs. Intel's energy-efficient processors are unfortunately limited to servers and laptops but AMD sells lines of energy-efficient desktop processors. The original 90 nm Athlon 64 X2 EE line has a 35- or 65-watt TDP versus an 89 W TDP for the standard chips and the newer 65 nm line Athlon BE line has a 45-watt TDP versus a 65-watt TDP for the standard chips. Currently-shipping AMD energy-efficient processors have an "e" suffix, such as the Athlon X2 4050e/4450e/4850e and the Phenom X4 9100e and 9300e. Energy-efficient Opterons have the "HE" suffix.
Running a laptop CPU in a desktop motherboard
Those of you who are even more adventurous might want to try running an efficient, low-power laptop CPU in a desktop motherboard in a "Mobile-on-Desktop" aka "MoD" setup. The best candidates for this setup are mobile CPUs that have the same socket as common desktop CPUs as motherboards are inexpensive and easy to find. Socket A Athlon XP-Ms and socket 754 Turion 64s are very popular to use in this manner. Socket 478 Mobile Pentium 4s and the lower-TDP Mobile Pentium 4-Ms can also be used, but there are occasionally issues with BIOS and SpeedStep support. There are a few motherboards with laptop-specific sockets like Socket 479, Socket M, Socket P, and Socket S1. They tend to be very expensive and hard to find.
Vibration from wobbly components can often cause irritating buzzing noises. Regularly give your fans a clean by blasting them with compressed air (be careful as you can overspeed smaller fans), and if vibration seems to be a problem you can dampen your fans by placing rubber or silicone washers between the fan and the mounting. Other moving components (such as hard drives and optical drives) should be secured mechanically well. You could consider mounting your drives in a dampening cage or using anti-vibration grommets. Antec uses the latter to great effect. I'm personally a big fan of the Cooler Master 620's spring-clip mechanism of mounting optical drives; these are screwed in on one side, and are forced into a good fit by a spring clip on the other side.
Related to vibration are stupidly fast optical drives running at 16x or 52x or something similarly daft. To watch DVDs and play CDs, you shouldn't need a drive faster than 1x DVD or 1xCD respectively. The DVD and CD specifications are written such that a DVD should playback video or video perfectly well at the drive's lowest available speed, YMMV.
As of this edit there are no optical drives on the market that are specifically advertised as being quiet or having quietness as a feature. Some drives have "quiet" firmware/utilities available which will limit the speed to a more reasonable and less deafening level. Since almost all drives require that their firmware be upgraded using Windows or DOS you may run into difficulty attempting this method with a Linux based computer running MythTV. You do have the option of putting a drive into a Windows based computer, applying the firmware alteration, and then putting it back in your Linux based computer (a variation on this theme is to always have a FreeDOS bootable partition). If you have the choice between an 8x DVD drive and a 16x DVD drive, choose the slower (8x) one. The only place MythTV needs fast disc access is when ripping a CD or DVD. You may also want to experiment with hdparm, setcd or speedcontrol; All claim to be able to reduce the maximum speed of optical drives. setcd may require a patch to work with newer drives, see debian bug 367008. SPCR has a forum thread on the subject. But there appears to be no definitive study of the comparative quietness of DVD/CD drives.
Keeping all those cables tidy can do wonders for airflow within your case, and do away with the need for more stupidly fast fans. You can use rounded IDE cables rather than the usual fat ribbons, or SATA if it's available to you, and you can tie bundles of power cables together using zip ties, braiding or spiral wrap (my personal favourite, since it's easy to apply and remove).
Don't forget The Art of Cable Folding for those with tight wallets. Properly folded cables can be better than rounded IDE cables.
Last but not least, you can also attempt to soundproof your case with acoustic material. This is essentially sticky-backed foam that does a lot to absorb noise from inside the case, although you'll need a case with enough free space inside to accommodate its thickness. I've never tried this myself, but a lot of people swear by it. Lian Li and Zalman make some very pretty (and pretty expensive) cases fitted with this sort of anti-noise technology as standard, so those of you with bigger wallets than myself who want a big, quiet case might want to consider splashing out on those.
And, even more finally (:-), you can uncouple the case mechanically from whatever is underneath it, or whatever it's underneath. Conducted noise which is then radiated by something else is a major component of the noise which a PC can put out — which explains the rubber mounts you can get for hard drives. They're good for noise, but bad for thermal conduction. (You do have a fan blowing directly on your hard drives, right?) Standing the chassis on a piece of carpet, or even better, underlay will cut down on transmitted noise through the floorboards, plasterboard etc.
That'll about do it for now, hope you enjoyed the update ;)
For lots of information on keeping your HTPC (and PCs in general) quiet the website Silent PC Review has some excellent guides, reviews and forums to provide you with all the details you need. Their recommended hardware section contains lots of hard numbers.
SilentPC Review - Contains reviews and guides on quiet PC cases, fans and components.