I need to understand how rewind TV works

***Saturlight*** · 26-10-2007

You may know that I bought a DVD HDD recorder. This I have connected to my satellite receiver and freeview box.

There are two options with the way to record. I can have the recorder on 6 hour timeshift which sort of records the live TV, so I can rewind, and fast forward whenever I like. I can then rewind, then record if I fancy something from that point. This can be done on the DVD or HDD.

The thing is, what I am trying to figure out, is how is it possible that I am able to rewind live TV back say 10 minutes and continue to watch from that point until the show is over 30 minutes later. That's 10 minutes after the programme has really finished.

If you rewind a tape, recording stops, how does this continue unterrupted?

**Topper** · 26-10-2007

Very basic and very simple

The hard drive has a read arms and a write arms, they are completely independent of each other therefore can work almost side by side but in reality I would imagine they are fixed opposite each other to balance loadings, there must be a minimum delay but I do not know what it is

below is a very simple illustration

**moosegoose** · 26-10-2007

basically it can record and play at the same time.im not very clever but i suppose it must have two lazers one for burning and one for reading.if im wrong someone tell me.

***2cvbloke*** · 26-10-2007

Originally Posted by Topper

Very basic and very simple

The hard drive has a read arms and a write arms, they are completely independent of each other therefore can work almost side by side but in reality I would imagine they are fixed opposite each other to balance loadings, there must be a minimum delay but I do not know what it is

below is a very simple illustration

Actually, no, a hard drive only has 1 arm onto which the Read/Write heads are fixed, this runs accross the surfaces of the disk platters, so for recording and playback, the heads are moving rapidly between writing and reading the platters...

Here's a photo of a HDD witht the top off...

**Topper** · 26-10-2007

I totally agree but to the non technically minded it is often easier to simplify the situation to make it easier for them to understand and it is a lot easier to draw, but then there's always someone wanting to complicate things

It would be interesting though to know how the reality is achieved since to perform both functions simultaneously you would think the arm would be continuously hunting whilst alternating between read and write functions

***2cvbloke*** · 26-10-2007

Originally Posted by Topper

I totally agree but to the non technically minded it is often easier to simplify the situation to make it easier for them to understand and it is a lot easier to draw, but then there's always someone wanting to complicate things

I was just stating the truth, you never know, someone might take a hard drive, open it up expecting to find 2 arms only to fing that they have 1 arm...

It would be interesting though to know how the reality is achieved since to perform both functions simultaneously you would think the arm would be continuously hunting whilst alternating between read and write functions

That's easy, a Memory Buffer, the drive reads the data, stores it in RAM, the receiver reads the incoming data, stores it in RAM, then the receiver plays back the Read data from RAM whilst the drive writes the incoming data from the RAM...

**Topper** · 26-10-2007

No that is not my point, that aspect is clear but in say a 15 minute delayed program, the arm will have to move to read a buffer full and then the arm will have to move back to write a buffer full thus creating a the hunting back and forth

***2cvbloke*** · 26-10-2007

Originally Posted by Topper

No that is not my point, that aspect is clear but in say a 15 minute delayed program, the arm will have to move to read a buffer full and then the arm will have to move back to write a buffer full thus creating a the hunting back and forth

Well, that is what they're designed to do...

If you think about it, a hard drive in a PVR has an easier job compared to one in a PC which has a tonne of data, software and media, not to mention the OS, piled onto it, so going back and forth between the writing & reading is pretty easy for the drive...

***Saturlight*** · 27-10-2007

It may not be technically clever by today's standards but I wish I had invented one in 1978. I'd be a millionaire by now.

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I need to understand how rewind TV works For those of you with an appetite for the scientific and technical, whether it be lofty theory or more practical discussion.