# Analog signal can be discrete time?

I am new to this site and field too.I always thought analog is continuous and digital signal is discrete. I read this today and got confused about analog signal

Analog can be continuous time(CT) or discrete time(DT). A discrete time signal is not a digital signal; a discrete time signal is one in which the amplitude is a continuum, and not discretized. On the other hand, if the time is discretized, this signal is still analog. Only when a discrete time signal is passed through an A to D converter, it becomes a digital signal.

Will anybody explain me what actually it means?

• filters can be made outa Charge-Coupled Devices (CCD) a.k.a. "bucket brigade" and would be an example of discrete-time and analog. Jan 2, 2015 at 19:43

This sounds like a confusion in terminology. See http://en.wikipedia.org/wiki/Digitizing

Digitization basically involves two steps:

1. Discretization: Sampling the signal at discrete times
2. Quantization: Turning the samples from (in theory) infinite resolution to finite resolution

Look at all possible combinations you can have 4 different types of signal:

1. continuous time, continous amplitude
2. discrete time, continous amplitude
3. continuous time, discrete amplitude
4. discrete time, discrete amplitude

Typically we refer to #1 as "analog" and #4 as "digital". #2 and #3 are of little practical relevance, other than getting the mathematical models right. Quantization and Discretization are really quite different mathematically so it doesn't make a lot of sense to analyze them in combination. In other words: the theory of time discretization applies equally to continuous amplitude and discrete amplitude signals.

• so whether #2 and #3 are combination of analog and digital signal? Jan 8, 2015 at 14:37
• An important case of "almost" #2 in your list is a high precision algorithm implementation, where the quantization is absolutely neglegible (of no consequence to the performance of the algorithm). This is typically a prototype implementation, which may later be transformed into a reduced wordlength implementation to reduce the hardware complexity (make it cheaper).
– Jens
Jan 26, 2015 at 19:54
• Real world #2 is en.wikipedia.org/wiki/Switched_capacitor May 22, 2017 at 20:54
• @user3857907 Take a look at this answer: dsp.stackexchange.com/a/34656/26009 May 22, 2017 at 22:43