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The development of the digital public switched telephone network (PSTN) involved the use of pulsed code modulation (PCM) and the T-carrier trunk hierarchy standards, which are fundamental to how digital voice communication is transmitted over the network.

Pulsed code modulation is a method used to convert an analog signal into a digital signal by sampling the audio and encoding it into a binary format. This technology allowed for clearer and more reliable transmission of voice signals over long distances, which is essential in a telephone network.

The T-carrier system, particularly in the United States, refers to a series of digital transmission systems that use time division multiplexing to combine multiple voice and data signals into a single digital signal. The T1 and T3 lines, for instance, are widely used in PSTN infrastructures to facilitate high-capacity communication.

These combined standards were pivotal during the transition from analog to digital telecommunications, enhancing the efficiency and capacity of the network. This context helps illustrate why the answer centered on pulsed code modulation and T-carrier trunk hierarchy is indeed the correct explanation in relation to the digital PSTN standards.