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Noob Question - Sub Woofer cable

Audiolab 6000A Amplifier

churfsan

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Hi all,

Now this may be a NOOB question.

I had been to a showroom in Ashok Metropolitan mall, it is above the Reliance Digital (in Hyderabad).

I saw one cable - it was marked as "DIGITAL COAXIAL CABLE" and it bore a stamp saying What Hi Fi Rated Four Star. It is priced at Rs. 1199/-. It was having RCA connector at both ends.

There was another pack, of the same company, which has a PAIR of cables, each having the same RCA connectors at both ends. This means that there are two cables, with RCA connector at both ends. It is from the same company, however it is marked as FOR AUDIO, no mention of DIGITAL. It is also priced at 1199/-, the connectors look same and the cables also look same. I could not understand the difference. It is giving two cables similar to the other pack at the same price as the price of the one cable, marked DIGITAL.

Hence the NOOB question:
Whether the coaxial cable connecting DVD to AVR, marked DIGITAL is the same as the cable which can be connected from the AVR to Sub-woofer (again coaxial)? Or is the Sub-woofer cable different from the DIGITAL coaxial cable?

Please help on this NOOB question.

Thank you very much
Churfsan
 

venkatcr

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The physical composition of an analogue coaxial and a digital coaxial are the same. But that is where the similarity ends.

Transmission of digital data, though also electrical, is a series of pulses that represent the '0's and '1's of digital data. Any interference, internal or external, could scramble the data, giving you noise at the other end. How external you may ask? When you place a digital coaxial cable next to a power cable, the power cable will be radiating EMI signals that could disturb the digital transmission.

Thus a digital coaxial cable has to have the following minimal characteristics:

  • Very low attenuation. In other words, the transmission will not lose data over reasonable lengths.
  • Impedance of 75 ohms. Analogue coaxial cables can vary in impedance between 30 to 100 ohms, though some of the high end cables for audio also have an impedance of 75 ohms.
  • Capacity to carry a wider bandwidth of data
  • Very high shielding. Coaxial audio cables may not need very high shielding as they are less prone to interference.

Thus the very construction of a digital coaxial cable will be different. The internal core copper cable will be of a higher quality to reduce attenuation and be capable of carrying weak signals. The shielding will be thicker giving a overall thicker cable.

That is the reason that digital coaxial cables are more expensive.

Cheers
 

churfsan

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The physical composition of an analogue coaxial and a digital coaxial are the same. But that is where the similarity ends.

Transmission of digital data, though also electrical, is a series of pulses that represent the '0's and '1's of digital data. Any interference, internal or external, could scramble the data, giving you noise at the other end. How external you may ask? When you place a digital coaxial cable next to a power cable, the power cable will be radiating EMI signals that could disturb the digital transmission.

Thus a digital coaxial cable has to have the following minimal characteristics:

  • Very low attenuation. In other words, the transmission will not lose data over reasonable lengths.
  • Impedance of 75 ohms. Analogue coaxial cables can vary in impedance between 30 to 100 ohms, though some of the high end cables for audio also have an impedance of 75 ohms.
  • Capacity to carry a wider bandwidth of data
  • Very high shielding. Coaxial audio cables may not need very high shielding as they are less prone to interference.

Thus the very construction of a digital coaxial cable will be different. The internal core copper cable will be of a higher quality to reduce attenuation and be capable of carrying weak signals. The shielding will be thicker giving a overall thicker cable.

That is the reason that digital coaxial cables are more expensive.

Cheers

Thank you venkatcr,

Does this mean that I can use Digital Co-axial cable for Sub-woofer connection (AVR to Sub-woofer)? If it is good enough for DIGITAL connection, will it also work satisfactorily for AVR to Sub-woofer connection also? Or do I need to use a separate special cable for AVR to Sub-woofer connection?

Thank you for your patience

Churfsan
 

ajinkya

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[*]Very high shielding. Coaxial audio cables may not need very high shielding as they are less prone to interference.
[/LIST]
Venkat,
This point is confusing to me. I thought the whole reason for us to go digital is because analog is more prone to interference whereas digital is a thresholding operation. Either you get a 0 or 1, assuming noise levels don't go above signal voltage levels. Same analog applies to AM transmissions being noisy and requiring more power compared to XM radio or even FM.

I am also not sure why digital coax will have weak signals being carried. I've seen HDMI cables many metres in length that work just fine...I assume coax will have similar properties. As an aside, does anyone know the voltage on an analog RCA vrs. on a digital coaxial cable?

Thanks,
Jinx.
 

marsilians

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Venkat,
This point is confusing to me. I thought the whole reason for us to go digital is because analog is more prone to interference whereas digital is a thresholding operation. Either you get a 0 or 1, assuming noise levels don't go above signal voltage levels. Same analog applies to AM transmissions being noisy and requiring more power compared to XM radio or even FM.

Amongst other things, digital signals are easier & cheaper to produce + they provide larger bandwidth than analog (hence you can do 5.1 on a single toslink or coax cable).

Coax cable by its inherent build structure is well insulated from transmission. So you don't see that much of an impact when using those.


As an aside, does anyone know the voltage on an analog RCA vrs. on a digital coaxial cable?


This question does not make sense. what are you trying to get at?

On an academic note,
Ohm's law states , V = IR V = Voltage, I = Current (amps) R = Resistance (Ohms) For Coax = 75 ohms. and with 220 - 240 volt circuits stepped down to 110 - 120 volts, current = 1.47 - 1.6 amps (so you dont feel a shock when you touch the end of a connnected cable !)

Re; cables, something called 'characteristic impedance' is used to measure the voltage. Plus, there are multiple impedances, cable impedance and connector impedance. The audio cables are lower impedence typically 50

All the rating says is that the cable is capable of presenting the impedance not that every signal will hit 75 or 50 ohms.
 
Last edited:

ajinkya

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Marsilians,

"Amongst other things, digital signals are easier & cheaper to produce + they provide larger bandwidth than analog (hence you can do 5.1 on a single toslink or coax cable).
Coax cable by its inherent build structure is well insulated from transmission. So you don't see that much of an impact when using those. "
That does not answer my first question which pertains to why digital should be more susceptible to interference than analog signals.

"1.47 - 1.6 amps (so you dont feel a shock when you touch the end of a connnected cable !)"
This is not true at all. This calculation completely ignores the series resistance of the human body from cable to ground. If you touch a coax cable that is live, then the electric circuit model should have the body resistance in series with the coax resistance of 75 ohms. Currents of 100 mA can be fatal, depending on what kind of body resistance we are measuring. See details here:
Electric Shock Hazards
Electric Shock Hazards

Electromagnetism is a fascinating and complex subject with many misconceptions still prevailing within our minds :) It took me a while to get my mind wrapped around why inductance arises in the first place.

Let me rephrase my question regarding the coax and RCA voltages:
* Are the voltage ranges of signals passing via digital coax and analog RCA the same? If not, does anyone know the magnitude of the ranges?
What I am referring to, I think is the concept of standard line-level voltage.

Thanks,
Jinx.
 

venkatcr

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The switch to digital form for audio and video has multiple reasons. Some of them are:

  • Storage - analogue storage (particularly) in tapes has a limited life. Magnetic surface degrade and lose the data. Though a vinyl can store data literally permanently, they also degrade with use. Digtal data, once stored, never changes.
  • Space constraints - Analogue information need huge space either as tapes for music or as film reels for movies. With the dramatic improvement in computer storage, hard disks are getting physically smaller with an ever increasing capacity to hold data.
  • Convergence - All forms of gadgets that you use have now have become compatible with one another. So why not use digital form of music and videos also to take advantage of this convergence?

But digital transmission, at least in real time, is not that easy as you will understand as your read.

Analogue signals are simple and represent the information they convey through a continuous waveform analogous to the information itself. A 500 Hertz tone, for example, is represented by a voltage varying from positive to negative and back again, at 500 times per second. This is called a sine-wave.

A digital signal, unfortunately, does not bear any resemblance to the information it is carrying. The logical '0's and '1's of digital data are represented as a rapid series of fluctuation or transitions in the voltage. These fluctuations are creating an instantaneous square wave. Thus the same 500 Hertz tone is represented differently, and is subject to interpretation at the receiving end.

There are two types of digital circuits. One is called Transistor-To-Transistor Logic or TTL. TTL circuits use bipolar transistors which are current-controlled devices. TTL circuits are restricted to voltages between 4.75 to 5.25 volts. TTL circuits are very fast, but need a constant regulated power supply of 5 volts. Any significant variations in that power supply voltage will result in the transistor bias currents being incorrect, which then results in unreliable or unpredictable operation.

The other digital circuit is CMOS and uses field effect transistors more commonly known as MOSFETs. CMOS circuits operate in either the saturated or cutoff modes and never in the active mode. Their inputs are, however, sensitive to high voltages generated by static electricity, and may even be activated into "high" (1) or "low" (0) states by spurious voltage sources. CMOS circuits can operate in any voltage from 3 to 15. CMOS circuits are easier to operate as they do not need precise voltages. The only effect of a voltage variation in a CMOS circuit is the logical conclusion arrived at. For example, if you use a 5 volt system with CMOS circuit, an voltage near to 5 will be considered as high or a logical 1. Similarly any voltage near to zero will be considered low or a logical 0. One disadvantage of CMSO circuits is their speed. They are slow, but this compensated by using buffers at the output signal to increase the overall voltage gain.

Generally systems followed is shown by the following example where VCC is the supply voltage.

CMOS
Low= 0V to VCC/2
High= VCC/2 to VCC

TTL
Low= 0V to 0.8V
High= 2V to VCC

But the voltage is not important for what we are discussing.

Binary representation are actually very easy to represent in physical terms. A binary bit can have only one of two different values - a '0' or a '1'. Any physical entity capable of switching between two states can be used to represent a bit.

One such entity are transistors in electronic circuits. When operated at their bias limits, transistors may be in one of two different states - either cutoff (no controlled current) or saturation (maximum controlled current). If a transistor circuit is designed to maximize the probability of falling into either one of these states, it can serve as a physical representation of a binary bit. A voltage signal measured at the output of such a circuit can also serve as a representation of a single bit - a low voltage representing a binary "0" and a relatively high voltage representing a binary "1."

But that is where the conundrum comes in. Let us say I design a digital circuit that has zero volt to represent logical 0, and 1 volt to represent logical 1. What happens when we get 0.5 volts? This kind of difference could come in because of various factors including attenuation in the cable or an external surge. A incorrect voltage would be invalid and would occur only in a fault condition or during a logic level transition, as most circuits are not purely resistive, and therefore cannot instantly change voltage levels. However, few logic circuits can detect such a fault, and most will just choose to interpret the signal randomly as either a 0 or a 1.

In a computer system, this does have too much of an effect. Digital transmission in computer circuits have what is called error correction in the form of a checksum carried at the end of a fixed quantum of data. The receiving circuit uses a program to calculate the checksum by itself from the data received and compares it with the checksum received. If there is a mismatch, the receiving circuit simply instructs the transmitting circuit to resend the data. This is done till the checksums match.

In audio and video circuitry, as they are in a real time, such checksum calculations or what is called error correction are difficult to perform. So when digital data are lost, the loss is final. Depending on what is lost, this may result in one of three options:

1. The receiving circuit will interpolate or guess the missing data.
2. The receiving circuit will completely misread the incoming data
3. The receiving circuit will completely miss the data, where the signals disappears for a time.

This will result in what we all know as jitter. As the digital signal has sharp transitions it is more sensitive to degradations as compared to a analogue signal.

In an analogue signal, the change of an analogue waveform is progressive and continuous--the more noise is introduced, the more noise will come out at the other end. Disturbance en route will not have such a serious effect on the waveform, and will be minor in nature.

A digital signal, on the other hand works differently. Because of its sharp transitions, it is highly sensitive and subject to sharp degradation in its waveform. A originating square wave will never arrive in it's original form. The corners of the square wave are rounded off to a greater or lesser degree, making the wave uneven. This makes it very difficult for the receiving circuit to correctly interpret the incoming signal and clock the signal. This is what we all know as jitter.

Jitter is introduced as the digital signal travels through the cable and depends upon the characteristic impedance of the cable, the capacitance, and the impedance match between the source and load devices.

Thus it is important to use a cable of high quality for digital circuits.

Cheers
 

venkatcr

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Does this mean that I can use Digital Co-axial cable for Sub-woofer connection (AVR to Sub-woofer)? If it is good enough for DIGITAL connection, will it also work satisfactorily for AVR to Sub-woofer connection also? Or do I need to use a separate special cable for AVR to Sub-woofer connection?

You have to use a digital coaxial cable for AMP to sub woofer. Actually there are special cables called -sub woofer' cables that are available for this task.

The strange point is that you can actually use digital coaxial cables for analogue connections, and they will provide better and more stable connectivity.

Cheers
 

marsilians

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Marsilians,

"1.47 - 1.6 amps (so you dont feel a shock when you touch the end of a connnected cable !)"
This is not true at all. This calculation completely ignores the series resistance of the human body from cable to ground. If you touch a coax cable that is live, then the electric circuit model should have the body resistance in series with the coax resistance of 75 ohms. Currents of 100 mA can be fatal, depending on what kind of body resistance we are measuring. See details here:
Electric Shock Hazards
Electric Shock Hazards

Electromagnetism is a fascinating and complex subject with many misconceptions still prevailing within our minds :) It took me a while to get my mind wrapped around why inductance arises in the first place.

Good point and absolutely my mistake - I should be careful in future postings. I fully ignored the body resistance. What was I doing during the electromagnetism classes ??? My mind is but a blur as it was influenced by somethings that rhyme with here, curls and dilema.

Let me rephrase my question regarding the coax and RCA voltages:
* Are the voltage ranges of signals passing via digital coax and analog RCA the same? If not, does anyone know the magnitude of the ranges?

No, speaking to a friend who works in power electronics industry (owns a factory who builds components), he says htat audio cables carry mA's worth of current where as speaker cables can go upto 8 A with inefficient speakers being driven to reference levels.


I know still no range. Where are the engineering grads? Come on guys, throw your 2 paise in.....
 
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