Monday, May 10, 2010

TiVo Wireless-N Adapter Arrives!

You can now get a wireless-N network adapter for your TiVo, $68.31 at the time of this writing at I have one on order and will report back later as to how well it works.

Wireless-N connections are nominally much faster than the wireless-G adapters TiVos have supported up to now. I now have one Series3 TiVo and two TiVo HDs. Each TiVo currently uses a wireless-G adapter.

My main interest in speeding things up is that I like to archive HD movies from any of my TiVos to my Mac, via my home wireless network, and then, when the mood strikes, stream one of them out to a TiVo for viewing. My wireless-G connections are too slow to stream HD in real time.

So, too, may be the wireless-N connection. I have made some experiments using wired Ethernet between the Apple AirPort Extreme base station that sits next to my Mac and my Series3 TiVo. I found that even wired-Ethernet speed can be too slow for delivering HD in real time ... and Ethernet is much faster than wireless-N.

But I also found a seeming difference between the Mac-to-TiVo-on-Ethenet transfer speed when I initiated a transfer but chose not to start playing it as it was in progress, as compared with when I chose to watch the video being transferred at the same time as it was being transferred. The latter scenario was much faster, making it seem, pending further experimentation, that the TiVo won't maximize the transfer speed unless the transferred recording is also being played during the process of making the transfer.

Also, there are several ways to do a Mac-to-TiVo transfer. When I made the experiment I just mentioned, I had yet to begin using pyTivoX (incorporating StreamBaby and FFmpeg) as the Mac-to-TiVo software engine on my Mac. I was instead using plain old TiVo Desktop. The particular transfer engine I use on my Mac may make a difference.

Stream remuxing

Another X factor is that copying a recording from a TiVo to a computer causes "remuxing" to take place. The TiVo "remuxes" (i.e., re-multiplexes) the recorded "transport stream" stored on its hard drive into a single MPEG-2 "program stream" that can be stored on a PC or Mac and played there.

I learned here, at the TiVo Community Forum, that:

High-definition recordings are stored on the TiVo’s hard drive as transport streams in a proprietary format. When you download a recording from the TiVo with a web browser (or TiVo Desktop), the TiVo remuxes the recorded streams stored on the hard drive into a single MPG file that can be played on a PC or Mac. This on-the-fly remuxing does not have any effect on quality, but it does cut throughput by 50-70% compared to MRV between two TiVos

When transferring recordings between two TivoHD DVRs, throughput is about twice as fast (20-24Mbps typical), because recorded files are transferred just as they are stored on the hard drive.

In a book I have, Charles Poynton’s Digital Video and HDTV, the author indicates that an MPEG-2 “transport stream” (TS) is comprised of a bunch of relatively small packets and is designed for “transmission of multiple programs on relatively error-prone media.” (That seems to describe to a T the transmission of cable TV to a TiVo.) “Multiple programs” would encompass, I’d assume, the video portion and the audio portion of a single cablecast. In a TS there are multiple independent “program clock references” that synchronize the separate programs. This would seem to be how video and audio are kept in sync.

In an MPEG-2 “program stream” (PS), which is designed for storage on relatively error-free media such as a computer hard drive, there are also packets, but they can be large: up to 64 KB in length, where a TS packet is only 188 bytes in length. Furthermore, synchronization in a PS is achieved through a “system clock reference,” not through program clock references.

So my guess is that in going from a TiVo to a computer, the recorded stream is converted by the TiVo from TS to PS — this is what "remuxing" means, I expect — which incurs CPU overhead on the TiVo and slows things down a lot. In going from the computer back to the TiVo, I’d assume the computer, whose CPU is relatively fast, takes responsibility for converting the PS back to a TS, and so the slowdown, if any, would not be as great.

Weak TiVo CPU power

Yet another X factor is the fact (see the TiVo Community Forum thread mentioned earlier) that the CPU of a TiVo has little CPU power, compared to a Mac or PC. It was not designed to handle more than about 75 Mbps of total throughput. Each HD stream consumes up to 20 Mbps, so if you are tuned to two HD channels and also watching a third HD recording that is already on your TiVo, the CPU overhead of two buffered HD channels and one in-progress HD playback can consume 55-60 Mbps. That worst-case scenario leaves 15-20 Mbps for handling MRV and PC transfers. During such transfers the CPU is responsible for, among other things, any remuxing that has to be done, so in a worst-case scenario such as this one the slowness of the TiVo's CPU can override an otherwise fast wireless-N or wired-Ethernet transfer speed, I assume.

In other words, there are several possible factors that can "bottleneck" a transfer from a TiVo to a computer — or in the reverse direction — such that the speed of the network connection is not the limiting factor. If this is the case, then upgrading from wireless-G to wireless-N might make little or no practical difference.

The same is not necessarily true, though, for TiVo multi-room viewing, since the need for the sending TiVo to remux the TS recorded on it is absent in that scenario.

Sidestepping wireless-G bottlenecks

However, in order to take advantage of potential wireless-N increase in MRV speeds, one needs to use a wireless-N adapter for both TiVos. A network path is only as fast as its slowest link. If there is a wireless-G link anywhere in the path between the two TiVos — or between any two devices on the wireless network — the network speed will be limited to that of wireless-G.

This is why anyone who gets a wireless-N adapter for a TiVo needs to be aware that the wireless router being used is a possible bottleneck. Say you have a router that is limited to wireless-G operation (or wireless-B operation, which has the same top speed as wireless-G). TiVo #1 can still MRV recordings from itself to TiVo # 2, even if both TiVos have a wireless-N adapter ... but the wireless-G router that is in the signal path between the two TiVos will slow the process down to wireless-G speed!

TiVo Inc. says the way around that is to hook an extra TiVo wireless-N adapter into an Ethernet port on the wireless-G router. That extra adapter acts as a "bridge" between any two outboard wireless-N adapters, hooked to their respective TiVos, and keeps the slowness of the wireless-G router from interfering with overall wireless-N transfer speeds.

I have a wireless-N capable router, though — an Apple AirPort Extreme base station using wireless-N operating mode. However, I also have been employing AirPort Express units, three of them, as part of a "wireless distribution network" (WDS) in my home. My particular AirPort Express units are limited to wireless-G operation. (Newer AirPort Express models are N-capable.) My base station formerly treated my AirPort Express units as range-extending parts of itself, so anything such as a TiVo that uses my home network might (possibly) access the network through a slow link: a wireless-G AirPort Express.

For now, I have simply unplugged my AirPort Express units and reconfigured my base station to stand alone in my home network. Having no WDS to extend the network range ensures that there cannot be a wireless-G unit in the signal path between any two of my TiVos or any one of them and my Mac.

Wireless-N adapter connection and configuration

The TiVo wireless-N adapter is unlike the wireless-G adapter in that the former hooks up to the Ethernet port on the back of the TiVo, not a USB port. (If you have an older TiVo model that lacks an Ethernet port, you're out of luck. Also, for reasons I am not sure of, the wireless-N adapter will work only on TiVos that have two tuners. If your TiVo has just one tuner, again you're out of luck. Nor will the wireless-N adapter work with any "DIRECTV DVR with TiVo" models.)

The TiVo wireless-N adapter must be plugged into an electrical power source such as a wall outlet or power strip. The wireless-G adapter draws its power from the USB port it plugs into. I judge from several recent posts to online forums that many potential adopters resent the wireless-N adapter's need to be plugged into external power.

The TiVo wireless-N adapter cannot be configured from the TiVo itself, as the wireless-G adapter can. You have to connect it to an Ethernet port on your computer, and also to a power source, to tell it what wireless network to use (even if you have only one such network) and also to enter the security phrase or password allowing connection via that network. Once you do that initial configuration, you can simply hook the wireless-N adapter to any TiVo.

If you have a router with "Wi-fi Protected Setup," it's even easier. You connect the wireless-N adapter via its Ethernet cable to that router (plugging the adapter into a power source as well) and press a button (on the router, I assume) that will set up the wireless-N adapter automatically. Then you move the wireless-N adapter to the TiVo.

My AirPort Extreme does not have this capability, so I can't test "Wi-fi Protected Setup."

More later ...

After I receive my wireless-N adapter and try it out, I'll report back on how it did ...

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