In the contemporary digital landscape, digital signage has transitioned from a mere advertising tool to an indispensable component of our urban infrastructure. From interactive terminals in metropolitan shopping districts to dynamic billboards and informative panels at international transit hubs, the omnipresence of digital signage is undeniable.

As we navigate this technological renaissance, it becomes imperative to delve deeper into the foundational principles that underpin the efficacy and relevance of digital signage. Herein, I elucidate the three quintessential pillars:

1. Content: The Heartbeat of Digital Signage

The efficacy of the Kitcast Apple TV signage app and other systems is intrinsically tied to the caliber of its content.

Dynamic Content Adaptation. Leveraging the power of real-time analytics and big data, digital signage must possess the capability to modify content based on instantaneous data. Envision a scenario where a digital billboard, cognizant of meteorological shifts, promotes winter wear during an unexpected cold front.

Interactivity and Engagement. Contemporary digital signage must transcend the traditional boundaries of unidirectional communication. By assimilating features such as touch-responsive interfaces, voice command recognition, and augmented reality overlays, we can metamorphose passive observers into engaged participants, thereby amplifying user immersion and interaction.

Cultural and Geographical Localization. It's paramount that content resonates with its immediate environment. A digital display in Paris should be imbued with the cultural subtleties and linguistic nuances that cater to its local demographic, mirroring the tailored approach of a counterpart in Mumbai.

2. Technological Symbiosis: The Backbone of Modern Signage

Digital signage realizes its zenith potential when harmoniously integrated with avant-garde technologies.

IoT and Urban Intelligence. In the era of interconnected urban ecosystems, digital signage should seamlessly interface with a myriad of devices. Consider a digital terminal at a railway station, synchronized with the city's rail network, offering passengers real-time schedule updates and platform changes.

AI-Driven Personalization. By amalgamating advanced camera systems with sophisticated AI algorithms, digital displays can discern viewer demographics, subsequently curating content that aligns with their preferences. This not only accentuates the user experience but also optimizes the precision and impact of promotional content.

Unwavering Connectivity. With the advent of 5G technology and its successors, digital signage must be equipped with robust, uninterrupted connectivity, facilitating real-time data retrieval, ultra-high-definition media streaming, and instantaneous content updates.

3. Sustainability and Ethical Governance: The Conscience of Digital Displays

The ubiquity of digital signage necessitates a conscientious approach towards environmental and ethical stewardship.

Energy Conservation. Continuous operational digital displays are significant energy consumers. Prioritizing energy-efficient display technologies, coupled with the integration of renewable energy-harnessing mechanisms like solar panels, can substantially reduce their ecological impact.

Data Privacy and Integrity. The convergence of AI-driven personalization introduces potential data privacy challenges. It's of paramount importance to ensure that personal data is processed with the utmost integrity, ensuring no unauthorized storage or misuse, while maintaining transparency with the audience regarding data utilization protocols.

Ethical Decommissioning. As the technological lifecycle progresses, outdated digital signage systems will inevitably be superseded. Instituting rigorous recycling and environmentally friendly disposal protocols ensures a sustainable approach to technological evolution.

In summation, the evolution of digital signage transcends mere technological advancements or aesthetic enhancements. It necessitates a comprehensive approach that seamlessly amalgamates superior content delivery, cutting-edge technological integration, and an unwavering allegiance to sustainability and ethical standards. As we navigate the future trajectory of this domain, these foundational pillars will undoubtedly dictate the refinement and relevance of digital signage, solidifying its position within our technologically-enriched milieu.

Moreover, at this juncture of technological progression juxtaposed with societal responsibility, it becomes imperative to recognize that the true merit of digital signage will be gauged not solely by its capacity to engage audiences, but equally by its commitment to overarching principles that champion collective welfare. The synergy of technology, ethical governance, and content strategy will be instrumental in shaping the future discourse of digital signage. 

Consequently, it is incumbent upon all stakeholders, encompassing manufacturers, content developers, and policymakers, to foster collaboration, drive innovation, and steadfastly uphold these cardinal principles, ensuring that digital signage remains a paragon of progress, adaptability, and ethical excellence in the forthcoming digital epoch.