MHz vs MT/s: How Should We Be Measuring RAM Speed?

MHz vs MT/s: How Should We Be Measuring RAM Speed?

The speed of Random Access Memory (RAM) is usually shown in megahertz (MHz) but this convention is being challenged with the case made that speed should be measured in mega transfers per second (MT/s). But what’s the difference, MHz vs MT/s, and which is best for accurately identifying the speed of RAM modules?

MHz (Megahertz)

Megahertz (MHz) measures frequency. Specifically, it measures the rate of repetition of a wave. One hertz is equal to one repetition (or cycle) per second, whilst mega means million so 1MHz is one million cycles per second. 

MHz is a unit of measure used in many fields, but within computing and electronics it has been used to measure the transmission speed within and between components, i.e. the rate of repetition of digital signals. The wave form is square offering the ability for computer components to recognise the wave pattern as binary code.

On_Off_Square_Wave_From

MT/s (Megatransfer)

One megatransfer is equal to the transfer of one megabyte. Measuring these megatransfers per second (MT/s) provides the amount of data which can be transferred per second. As such this actually measures the speed of the RAMs transfer capabilities. 

Measuring Frequency not Speed

Modern storage devices use transistors which switch the voltage on and off creating the square wave which the CPU can read. The frequency in MHz measures how quickly the transistors can switch the voltage on and off per second, often known as the clock speed. 

When RAM was first introduced, using MHz to denote speeds made sense as only one data transfer could be performed per clock cycle so the MHz and the MT/s were the same. A 600MHz clock speed would offer 600MT/s. 

However, since 2000 and the introduction of Double Data Rate (DDR) RAM modules it has been possible for data to be transferred on both the rising and falling signals of the square wave, essentially doubling the data transfers. Now a 600MHz clock speed offers 1,200MT/s.

SDR_vs_DDR

What’s the Issue? Why not just switch to MT/s?

Both accurately measure the capabilities of RAM memory, but the issue arises in how manufacturers are using the numbers. As measuring RAM speeds in MHz was the norm when DDR was introduced manufacturers wished to keep consistency for their customers and avoid confusion. Instead of switching to MT/s to denote speed manufacturers kept reporting in the understood standard (MHz). 

A RAM module with 1600MHz clock cycle and 3,200MT/s data rate may be advertised as 3,200MHz, which is technically incorrect. 

The practice has continued to this day. Some manufacturers fear the switch will make their RAM modules look half as fast without any change in specification. 

However, as people now have more access to technical information (like this explainer) the worries surrounding confusing customers is beginning to disappear. Many manufacturers (Integral included) are now reporting both the accurate clock speed and the data rate, instead of the manipulated clock speed.

 For those looking to purchase new RAM memory modules, both MHz and MT/s are ideal for measuring the speed and capabilities.

Laptop SO-DIMM

Laptop SO-DIMM

Laptop SO-DIMM

Laptop SO-DIMM

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