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In telecommunications, the carrier-to-noise ratio, often written CNR or C/N, is the signal-to-noise ratio (SNR) of a modulated signal. The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analog base band message signal after demodulation, for example an audio frequency analog message signal. If this distinction is not necessary, the term SNR is often used instead of CNR, with the same definition.

C - Pointers. As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined −.

Digitally modulated signals (e.g. QAM or PSK) are basically made of two CW carriers (the I and Q components, which are out-of-phase carriers). In fact, the information (bits or symbols) is carried by given combinations of phase and/or amplitude of the I and Q components. It is for this reason that, in the context of digital modulations, digitally modulated signals are usually referred to as carriers. Zimmer ats 1200 operator's manual. Therefore, the term carrier-to-noise-ratio (CNR), instead of signal-to-noise-ratio (SNR) is preferred to express the signal quality when the signal has been digitally modulated.

High C/N ratios provide good quality of reception, for example low bit error rate (BER) of a digital message signal, or high SNR of an analog message signal.

## Definition

The carrier-to-noise ratio is defined as the ratio of the received modulated carrier signal powerC to the received noise power N after the receiver filters:

${displaystyle mathrm {CNR} ={frac {C}{N}}}$.

When both carrier and noise are measured across the same impedance, this ratio can equivalently be given as:

${displaystyle mathrm {CNR} =left({frac {V_{C}}{V_{N}}}right)^{2}}$,

where ${displaystyle V_{C}}$ and ${displaystyle V_{N}}$ are the root mean square (RMS) voltage levels of the carrier signal and noise respectively.

C/N ratios are often specified in decibels (dB):

${displaystyle mathrm {CNR_{dB}} =10log _{10}left({frac {C}{N}}right)=C_{dBm}-N_{dBm}}$

or in term of voltage:

${displaystyle mathrm {CNR_{dB}} =10log _{10}left({frac {V_{C}}{V_{N}}}right)^{2}=20log _{10}left({frac {V_{C}}{V_{N}}}right)}$

## Measurements and estimation

The C/N ratio is measured in a manner similar to the way the signal-to-noise ratio (S/N) is measured, and both specifications give an indication of the quality of a communications channel.

In the famous Shannon–Hartley theorem, the C/N ratio is equivalent to the S/N ratio. The C/N ratio resembles the carrier-to-interference ratio (C/I, CIR), and the carrier-to-noise-and-interference ratio, C/(N+I) or CNIR.

C/N estimators are needed to optimize the receiver performance.[1] Typically, it is easier to measure the total power than the ratio of signal power to noise power (or noise power spectral density), and that is why CNR estimation techniques are timely and important.

## Carrier-to-noise density ratio

In satellite communications, carrier-to-noise-density ratio (C/N0) is the ratio of the carrierpowerC to the noise power densityN0, expressed in dB-Hz.When considering only the receiver as a source of noise, it is called carrier-to-receiver-noise-density ratio.

It determines whether a receiver can lock on to the carrier and if the information encoded in the signal can be retrieved, given the amount of noise present in the received signal. The carrier-to-receiver noise density ratio is usually expressed in dBHz.

The noise power density, N0=kT, is the receiver noise power per hertz, which can be written in terms of the Boltzmann constantk (in joules per kelvin) and the noise temperatureT (in kelvins).

• C/I: carrier-to-interference ratio
• Eb/N0 (energy per bit relative to noise power spectral density)
• Es/N0 (energy per symbol relative to noise power spectral density)
• Signal-to-interference ratio (SIR or S/I)
• Signal-to-noise ratio (SNR or S/N)
• SINAD (ratio of signal-plus-noise-plus-distortion to noise-plus-distortion)

## References

1. ^Islam, A. K. M. Najmul; Lohan, E. S.; Renfors, M. (Mar 2008). Moment based CNR estimators for BOC/BPSK modulated signal for Galileo/GPS. pp. 129–136.

This article incorporates public domain material from the General Services Administration document 'Federal Standard 1037C' (in support of MIL-STD-188).

• Digital Transmission: Carrier-to-Noise Ratio, Signal-to-Noise Ratio, and Modulation Error Ratio at the Wayback Machine (archived 2016-03-04)
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Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined −

When the above code is compiled and executed, it produces the following result −

## What are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is −

Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * used to declare a pointer is the same asterisk used for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Take a look at some of the valid pointer declarations −

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

## How to Use Pointers?

There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. The following example makes use of these operations −

When the above code is compiled and executed, it produces the following result −

## NULL Pointers

It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program −

When the above code is compiled and executed, it produces the following result −

In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

To check for a null pointer, you can use an 'if' statement as follows −

## Pointers in Detail

Pointers have many but easy concepts and they are very important to C programming. The following important pointer concepts should be clear to any C programmer −

Sr.No.Concept & Description
1Pointer arithmetic

There are four arithmetic operators that can be used in pointers: ++, --, +, -

2Array of pointers

You can define arrays to hold a number of pointers.

3Pointer to pointer

C allows you to have pointer on a pointer and so on.

4Passing pointers to functions in C

Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function.

5Return pointer from functions in C

C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well.